US20100147835A1

US20100147835A1 - Doped Gallium Nitride Annealing

Info

Publication number: US20100147835A1
Application number: US12/463,952
Authority: US
Inventors: Rao V. Mulpuri; Yonglai Tian; Siddarth G. Sundaresan
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-05-09
Filing date: 2009-05-11
Publication date: 2010-06-17
Also published as: US20100068871A1; US7994027B2

Abstract

The present invention involves annealing methods for doped gallium nitride (GaN). In one embodiment, one method includes placing, within a heating unit, a silicon carbide (SiC) wafer as a susceptor in close proximity with a doped GaN epilayer, wherein the doped GaN epilayer is either a GaN layer grown on a substrate or a GaN layer that is free standing; and heating, at a heating rate of at least about 100° C./s, the wafer and the doped GaN epilayer to at least about 1200° C. In another embodiment, another method includes placing, within a heating unit, a doped GaN epilayer, wherein the doped GaN epilayer is either a GaN layer grown on a conducting substrate or a GaN layer that is free standing; and heating, at a heating rate of at least about 100° C./s, the doped GaN epilayer to at least about 1200° C.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of provisional patent application Ser. No. 61/051,902 to Sundaresan et al., filed on May 9, 2008, entitled “Nanowire Growth Using Microwave Heating-Assisted Physical Vapor Transport,” which is hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grant W911NF-04-1-0428 awarded by the Army Research Office; SBIR grant no. 0539321 awarded by National Science Foundation (NSF); grant no. DMR 05-20471 awarded by NSF UMD MRSEC; and award nos. ECS-0618948 and ECCS-0742139 both awarded by NSF. The government has certain rights in the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow diagram of an annealing method for doped GaN as one aspect of the present invention.
FIG. 2 shows a flow diagram of an annealing method for doped GaN as another aspect of the present invention.
FIG. 3 shows an example of a block diagram of a solid-state microwave annealing system.
FIG. 4 shows another example of a block diagram of a solid-state microwave annealing system.
FIG. 5 shows examples of XPS survey scans of: (a) AlN as-capped GaN sample, (b) AlN capped sample after 1400° C./5 s annealing, and (c) after removal of the AlN cap at the conclusion of annealing.
FIG. 6 shows an example of low-temperature (5 K) PL spectra of both as-grown Mg-doped GaN and AlN capped samples subjected to 5 s microwave annealing at 1300° C. and 1500° C.
FIG. 7 shows an example of low-temperature (5 K) PL spectra of as-grown Mg-doped GaN and e-beam deposited MgO capped in-situ Mg-doped GaN samples after 5 s microwave annealing at 1300° C. and 1350° C.
FIG. 8 shows an example of hole concentration as a function of annealing temperature for 5 s duration microwave annealing on uncapped, MgO capped, and AlN capped in-situ Mg-doped GaN.
FIG. 9 shows an example of a comparison between simulated and experimental (as-implanted) Mg multiple energy implant profile in GaN.
FIG. 10 shows examples of SIMS depth profiles of the Mg implanted GaN before and after 1300° C./5 s and 1400° C./5 s microwave annealing.
FIG. 11 shows an example of low-temperature (5 K) PL spectra from an un-implanted GaN epilayer, and GaN epilayers before and after 1400° C./5 s and 1500° C./5 s microwave annealing.
FIG. 12 shows an example of XRD spectra of the multiple-energy Mg ion-implanted and the microwave annealed samples for 5 s duration.
FIG. 13 shows an example of XRD scans of the GaN (004) for single energy Mg as-implanted and microwave annealed (15 s duration) films.
FIG. 14 shows an example of low-temperature (5 K) PL spectra from an un-implanted GaN epilayer, and multiple energy Mg-implanted GaN epilayers before and after 1400° C./5 s and 1500° C./5 s microwave annealing.
FIG. 15 shows an example of low-temperature (5 K) PL spectra of single-energy Mg:GaN film grown on 4H—SiC; and of AlN-capped samples subjected to 15 s microwave annealing at 1300° C., 1400° C., and 1500° C.
FIG. 16 shows an example of two-probe I-V measurements on the single energy Mg-implanted, 15 s microwave annealed samples.

DETAILED DESCRIPTION OF THE INVENTION

The present invention embodies novel heating techniques for dopant annealing. In one embodiment, discloses annealing methods for doped gallium nitride (GaN). In one instance, the doped GaN is grown on a nonconducting substrate. In another instance, the doped GaN is grown on a conducting SiC substrate. In yet another embodiment, dopant annealing may be applied in semiconducting materials.
Referring to FIG. 1, a doped GaN annealing method is shown. As one embodiment of this method comprises placing, within a heating unit, a SiC wafer as a susceptor in close proximity with a doped GaN epilayer. The doped GaN epilayer may either be a GaN layer that is grown on a substrate or a GaN layer that is free standing S105. The thickness of the GaN layer may be less than or equal to about 1 mm.
Close proximity means either one element (i.e., SiC wafer) touching another element (i.e., GaN epilayer) or at most a distance of about 500 micrometers. Furthermore, the SiC wafer may be located above, below, near, or to the side of the doped GaN epilayer. The SiC may be a hexagonal SiC (e.g., 4H—SiC, 6H—SiC, etc.) or cubic SiC (e.g., 3H—SiC). The purpose of using SiC as a susceptor is because SiC acts as a good absorber of microwaves and consequently heats up rapidly. In other words, the SiC wafer as a conduit for the heating (annealing) of the epilayer.
The substrate used may either be a nonconducting substrate or a conducting substrate. Where a nonconducting substrate is used, the nonconducting substrate may be a semi-insulating substrate (e.g., SiC, etc.) or an insulating substrate (e.g., sapphire, etc.). Where a conducting substrate is used, the conducting substrate may be a doped SiC substrate, a conducting GaN substrate, or any other conducting substrate. After such placement, the wafer and the epilayer are heated to at least about 1200° C. S110. The rate of heating is at least about 100° C./s.
The doped GaN may either be in situ doped or ion-implantation doped. The dopant used for the doped GaN can be magnesium, beryllium, calcium, zinc, silicon, sulfur, iron, cobalt, vanadium, or any combination of these elements.
The method may further include depositing a protective capping layer on the GaN epilayer. In one embodiment, the protective capping layer is aluminum nitride (AlN). AlN may be deposited using pulsed-laser deposition.
In another embodiment, the protective capping layer is magnesium oxide (MgO). The MgO may be deposited using electron beam evaporation of an MgO target. The MgO target may be fused lumps of MgO, wherein the fused lumps comprise of a multitude of about 3 to about 12 mm pieces of 99.95% metals basics.
The method may be performed in a vacuum chamber or an inert gas atmosphere (e.g., helium, neon, argon, krypton, or xenon).
The method can use any kind of known heating unit. For example, the heating unit may be a microwave heating head (such as the Microwave RTP System by LT Technologies of Fairfax, Va.). In another example, the heating unit may be a laser annealing system (such as LA-TF or Laser Anneal 6300, both by AMBP Tech of Piscataway, N.J.). In yet another example, the heating unit may be a halogen or mercury lamp.
While these heating units are known, the present invention's doped GaN annealing techniques remain novel. It is within the scope of the present invention that such annealing techniques include, but are not limited to, microwave heating annealing, laser annealing, halogen lamp annealing, mercury lamp annealing, etc. Although the following embodiments use microwave heating annealing as an example, any of these exemplified techniques may be used to achieve the requisite high heating rates and temperatures of the present invention.
Referring to FIG. 2, another doped GaN annealing method is shown. However, the method here does not involve a susceptor or a nonconducting substrate. Rather, the method comprises placing, within a heating unit, a doped GaN epilayer S205. The doped GaN epilayer may either be a GaN layer that is grown on a substrate or a GaN layer that is free standing S105. The substrate used may be a conducting, insulating, or semi-insulating GaN substrate. The thickness of the GaN layer may be less than or equal to about 1 mm. Afterwards, the epilayer is heated to at least about 1200° C. S210. The rate of heating is at least about 100° C./s.
The doped GaN may either be in situ doped or ion-implantation doped. The dopant used for doping GaN can be magnesium, beryllium, calcium, zinc, silicon, sulfur, iron, cobalt, vanadium, or any combination of these elements.
Furthermore, this method may also include depositing a protective capping layer on the GaN epilayer. Like above, as one embodiment, the protective capping layer is AlN. AlN may be deposited using pulsed-laser deposition. In another embodiment, the protective capping layer is MgO, where the MgO may be deposited using electron beam evaporation of an MgO target. The MgO target may be fused lumps of MgO, wherein the fused lumps comprise of a multitude of about 3 to about 12 mm pieces of 99.95% metals basics. Moreover, the method may be performed in a vacuum chamber or an inert gas atmosphere (e.g., helium, neon, argon, krypton, or xenon).
Similarly, this method can use any kind of known heating unit. For example, the heating unit may be a microwave heating head (such as the Microwave RTP System by LT Technology of Fairfax, Va.). In another example, the heating unit may be a laser annealing system (such as LA-TF or Laser Anneal 6300, both by AMBP Tech of Piscataway, N.J.). In yet another example, the heating unit may be a halogen or xenon lamp.
While these heating units are known, the present invention's doped GaN annealing techniques remain novel. It is within the scope of the present invention that such annealing techniques include, but is not limited to, microwave heating annealing, laser annealing, halogen lamp annealing, mercury lamp annealing, etc. Although the following embodiments use microwave heating annealing as an example, any of these exemplified techniques may be used to achieve the requisite high heating rates and temperatures of the present invention.
I. Introduction
GaN belongs to a class of semiconductors known as wide band-gap semiconductors. GaN is important for high-power solid-state devices, especially for those intended for microwave frequency range and also for optoelectronics applications on account of its direct band gap. The GaN based high electron mobility transistors (HEMTs) have defined state-of-the-art for output power density and have the potential to replace GaAs based transistors for a number of high-power applications. The advantages of GaN over other semiconductors include: a high breakdown field (3 MV/cm, which is ten times larger than that of GaAs), a high saturation electron velocity (2.5×107 cm/s), and the capacity of the III-nitride material system to support heterostructure device technology with a high two-dimensional electron gas (popularly known as 2DEG) density and high carrier mobility.
Another attractive feature of all III-nitride semiconductors is the possible polarization-induced bulk three-dimensional doping without physically introducing shallow donors. The strong piezoelectric effect and a large spontaneous polarization in the III-nitride system allows for the incorporation of a large electric field (>106 V/cm) and a high sheet charge density (>1013 cm-2) without doping. This effect and polarization help to realize a variety of high-performance and high-power microwave devices.
As indicated in the following table, TABLE 1 shows a comparison of material properties of several semiconductors, namely Si, GaAs, SiC, and GaN.

TABLE 1

Material Properties of Semiconductors Si, GaAs, and GaN

Attribute	Si	GaAs	GaN

Energy Gap (eV)	1.11	1.43	3.4
Breakdown E-Field (V/cm)	6.0 × 10⁵	6.5 × 10⁵	3.5 × 10⁶
Saturation Velocity (cm/s)	1.0 × 10⁷	2.0 × 10⁷	2.5 × 10⁷
Electron Mobility	1350	6000	1600*
(cm²/V-s)
Thermal Conductivity	1.5	0.46	1.7
(W/cmK)
Heterostructures	SiGe/Si	AlGaAs/GaAs	AlGaN/GaN
		InGaP/GaAs	InGaN/GaN
		AlGaAs/InGaAs

*Typical two-dimensional electron gas mobility for AlGaN/GaN heterostructures.

Ion-implantation is an indispensable technique for selective area doping of GaN, for fabricating high-power electronic and opto-electronic devices. Other doping methods, such as thermal diffusion, are impractical for GaN technologies because the diffusion co-effecients of the technologically relevant dopants in GaN is very small, even at temperatures in excess of 1800° C.
However, being a highly energetic process, ion-implantation can damage the semiconductor crystal lattice. Moreover, the as-implanted dopants tend not to reside in electrically active substitutional sites in the semiconductor lattice. Therefore, ion-implantation always needs to be followed by a high-temperature annealing step for alleviating the implantation-induced lattice damage and for activating the implanted dopants (i.e., moving them from interstitial to electrically active substitutional lattice sites).
For implanted n-type dopants (e.g., Si) in GaN, annealing temperatures in the range of 1200° C. are required, whereas implanted p-type dopants (e.g., Mg and Be) in GaN require annealing temperatures in excess of 1300° C. for (1) satisfactorily removing implantation-induced lattice damage, (2) activating the implanted dopants, and (3) recovering the luminescence properties (which are severely degraded by the ion-implantation). The higher temperature requirement for activating p-type implants compared to n-type implants in GaN is primarily due to the much larger formation energy of the substitutional Mg_Gaspecies compared to the Si_Gaspecies.
Temperatures over 1300° C. are required for completely activating in-situ, as well as ion-implanted p-type dopants. However, when annealed at temperatures above 800° C., GaN decomposes into Ga droplets due to the nitrogen leaving the surface. Annealing of GaN can be performed in halogen lamp-based RTA systems because of the rapid heating/cooling rates accorded by these RTA systems. However, due to their quartz hardware, these halogen lamp based RTA systems are limited to a maximum temperature of 1200° C., which is not sufficient to effectively anneal p-type GaN.
A way to overcome these problems is using SSM heating. SSM heating is advantageous for high-temperature processing of wide-bandgap semiconductors, such GaN. This heating process has a capability to reach sample temperatures >2000° C. (for SiC wafers) with heating and cooling rates in excess of 600° C./s.
An example of a SSM RTP system 301 used in this work is illustrated in FIG. 3 and FIG. 4. This SSM RTP system has three main building blocks: (1) a variable frequency microwave solid state power source 305, which consists of a signal generator 310 and a power amplifier 315, (2) a microwave heating system 320, which consists of a coupling and tuning circuit 325 and at least one microwave heating head 330, 415, for coupling microwave power to the targeted source wafer having a T1 405 and (3) a measurement and control system 335, which consists of a network analyzer 340, a computer 345, an optical pyrometer 350, and other equipment. Below the targeted source wafer having a T1 405 and separated by a small gap is the substrate wafer having a T2 410. The temperature difference between T1 and T2 forms the temperature gradient ΔT. Microwave power generated by the variable frequency power source 305 is amplified and then coupled to a SiC sample 355, 410 through the microwave heating head 330. The environmental gas and pressure of the chamber can be controlled by vacuum pump 425 and external vapor/gas source 420. The sample temperature can be monitored through a viewport 430 by an infrared pyrometer or the optical pyrometer 350. The SiC and GaN sample emissivities can be measured using a blackbody source. The measured emissivity value (e.g., 0.8) is then keyed into the pyrometer for all temperature measurements.
In the case of annealing doped GaN, either 405 or 410 can represent the doped GaN sample. The other one that is not represented by GaN may be a SiC susceptor. The susceptor may or may not be required.
The microwave system 415 above can be tuned to efficiently heat semiconductor samples at variable frequencies. For instance, operating at about 150 W, the frequency may range from about 910 MHz to about 930 MHz. Temperature may be maintained at a steady state, such as ˜1800° C., for a certain amount of time, such as about 15 seconds.
Since samples should be placed in an enclosure made of microwave transparent, high-temperature stable ceramics (such as boron nitride and mullite), microwaves only heat the strong microwave absorbing (electrically conductive) semiconducting films, which present a very low thermal mass in comparison with a conventional furnace where the surroundings of the sample are also heated. Thus, heating rates >600° C./s are possible.
Microwave heating has an advantage of selective heating. When microwave power radiates on two different materials, such as a highly doped SiC wafer (which tends to be a strong microwave absorber), and a semi-insulating SiC wafer (which tens to be a poor microwave absorber), microwaves may selectively heat up highly doped SiC of the strong microwave absorber while leaving a negligible direct, heating effect on the semi-insulating SiC.
Selective heating of thin, highly doped semiconductor layers is possible if the doped layers are formed on semi-insulating or insulating substrates. Therefore, for efficient microwave annealing of implanted semi-insulating (SI) SiC substrates and GaN epilayers grown on (electrically insulating) sapphire substrates, a 5 mm×5 mm heavily doped 4H—SiC sample can be placed as a susceptor directly underneath the sample to be annealed. It is possible to directly couple microwave power and heat GaN epilayers grown on sapphire, without using any susceptor. However, the spatial distribution of temperature across the sample may be extremely non-uniform. Placing a SiC susceptor sample underneath the GaN sample of interest can solve this problem.
III. Microwave Annealing of In-Situ and Ion-Implanted Acceptor Doped GaN
A. Problems
GaN is a very important (direct) wide bandgap semiconductor for fabricating opto-electronic devices in the short-wavelength region and for high-power/frequency devices. Reliable, planar, and selective area acceptor doping technology is required for making high performance GaN devices. Ion-implantation is the only post growth doping method available for this purpose in GaN, as thermal diffusion of the dopants is not feasible in this material owing to low nitrogen dissociation temperature (˜900° C.).
Like SiC, the inability to achieve high p-type conduction in GaN has so far limited the commercialization of this otherwise promising semiconductor for many electronic and opto-electronic applications. Similar to the strong Si—C covalent bond in SiC, the large ionicity of the Ga—N bond gives GaN its unique properties and makes it a difficult material to work with technologically. TABLE 2 lists the best R_svalues and other electrical properties reported to-date for p-type GaN.

TABLE 2

List of the Electrical Characteristics of p-type GaN Available from Literature

Specie and
doping	Dose	Depth	RTA	Sheet Res.	Mobility	Sheet Carrier
method	cm⁻²	μm	temp/time	Ω/□	cm²/Vs	Conc. cm⁻²

Be implant	1 × 10¹³	0.25	1100° C./30 s	8.4 × 10⁴	4.4	5.75 × 10¹³
Be implant	5 × 10¹⁴	40 KeV	PLA + RTA	8.4 × 10⁴	8.7	8.56 × 10¹²
			1100° C./120 s
Be/N = 1	2.5 × 10¹⁴	0.5	1200° C./10 s	5.6 × 10⁴	6.8	2.7 × 10¹²
Co-implant
Mg/N = 2	1.5 × 10¹⁵	0.3	1200° C./10 s	5.6 × 10³	7.7	1.4 × 10¹⁴
Co-implant

The minimum achievable R_sfor doping with magnesium (Mg) and beryllium (Be), the popularly used p-type dopants in GaN, is in the range 10⁴Ω/□-10⁵Ω/□. It should be noted that throughout the present invention, the unit Ω/□ means Ω/sq. Clearly, these values are too high to permit one to fabricate high-performance electronic and opto-electronic devices. The difficulty in achieving low sheet resistance for p-type GaN may be attributed to the presence of high densities of donor-type point defects such as nitrogen vacancies (V_N), and their complexes with native defects and acceptor dopants, which have relatively low formation energies. These defects are known to have a donor behavior in GaN, thus restricting the maximum p-type conduction. The achievement of high p-type conductivity is even more difficult in ion-implanted GaN layers because the implantation-induced damage creates extra donor-type defects, which compensate the activated holes. Moreover, the optical properties of GaN are greatly diminished by ion-implantation resulting in a complete loss of luminescence even for low doses. The strongest effect is the creation of non-radiative recombination centers due to the implant-induced damage. The introduced defects have mainly deep levels within the bandgap; therefore, the as-implanted GaN is electrically highly resistive. The damage must be annealed out to achieve optical and electrical activation of the implanted dopants.
Post implantation high-temperature (>1200° C.) annealing is required for removing the implantation generated defects as well as to activate the implanted dopants. Conventional annealing of GaN cannot be done at temperatures >900° C. for more than a few seconds, due to a low dissociation temperature (900° C.) of N. The low ramping rate of conventional annealing furnace also subjects the GaN samples to a much-prolonged unintentional heating, which may further introduce defects in the GaN films. Hence, to preserve the surface morphology and the lattice quality, the required annealing temperature should be reached very fast and the duration of anneal should be limited to few seconds.
The fundamental diffusion, recovery, and activation processes that occur in ion-implanted impurities in GaN as a function of annealing temperature are generally known. Optical activation can be achieved in the temperature range 1200° C.-1300° C. But defect complexes (which cause an intense yellow band in the photoluminescence (PL) spectra) require still higher temperatures (in excess of 1300° C.) to break up. Acceptor-type activation in GaN is much more difficult to achieve compared to donor-type activation due to presence of unintentional compensating deep donors (e.g., nitrogen vacancies, V_N, and its complexes), as well as the absence of shallow acceptors. Halogen-lamp based rapid thermal annealing (RTA) is a common method used to anneal the defects, to repair the lattice damage and to activate the dopants in GaN. The lowest R_svalues, 5.6×10⁴Ω/□ for Be-implanted GaN, and 5.6×10³Ω/□ for Mg implanted GaN in TABLE 2, were achieved using RTA at an annealing temperature of 1200° C. and a short anneal duration of 10 s.
AFM micrograph of a Be⁺-implanted GaN sample annealed by halogen-lamp RTA at 1100° C. for 2 min can be taken. The RMS roughness extracted from the AFM image may be 2 nm, despite only scanning a 1 μm×1 μm square area.
High-resolution x-ray diffraction spectra θ-2θ may also be recorded for (a) a MBE as-grown GaN sample, (b) a Be-implanted sample after combination of PLA and RTA, (c) a Be-implanted sample after RTA, (d) a Be-implanted sample after PLA, and (e) a Be-implanted sample without annealing. Compared to the MBE as-grown sample, the Be as-implanted sample generally has a broadening of the main (0002) GaN peak and the appearance of an additional peak at the low-angle side, which is consistent with the expansion of the GaN lattice in the implanted region. Even after the PLA treatment, the lower angle peak remains. This presence indicates that the shallow penetration depth of the 248 nm laser only anneals the near surface region and leaves the deeper crystal defects untouched. After RTA at 1100° C. following the PLA, the lower angle peak has disappeared. However, the main peak is still much broader compared to the as-grown sample, indicating that the RTA temperature is not high enough to anneal out all the implant-induced defects. Some residual strain still exists in the GaN layer after annealing. This result emphasizes the need for a RTA technique with a higher temperature capability.
Key requirements for optimum annealing conditions for GaN appear to include both a high annealing temperature and a short annealing time. As a rule of thumb, an implanted semiconductor should be annealed up to a temperature of ⅔ of its melting point for damage recovery and dopant activation. In the case of GaN, this temperature is about 1650° C. However, such high temperatures are well beyond the capability of most commercial halogen lamp-based RTA equipment, which only have a modest temperature capability of <1200° C. As such, an annealing method with the capability of high processing temperature (≧1300° C.) is needed. Compared to SiC, an additional difficulty arises in case of GaN. GaN cannot withstand slow heating rates and long duration anneals at temperatures ≧1000° C. because of an incongruent sublimation of GaN, which decomposes into a N-rich gas and a gallium rich liquid at higher temperatures. Hence, to preserve the chemical integrity of GaN, while simultaneously reducing the density of compensating defects, the required anneal temperature should be reached very fast and the annealing duration should be limited to a few seconds. Furthermore, compared to SiC, the anneal duration for GaN has to be shorter to preserve surface integrity. This requirement on ultra-fast heating rates is not met by most conventional equipment.
There exists an RTP unit based on a MOCVD system has been built that employs RF heating with heating rates of 50° C./s. It supposedly anneals ion-implanted GaN, capped with a layer of aluminum nitride (AlN) in the temperature range 1200-1500° C. It has been reported that temperatures as high as 1400° C. are required for alleviating the implantation induced lattice damage and optimally activating the implanted dopants.
The sheet carrier concentration and mobility of Si-implanted GaN subjected to annealing using such existing RTP unit may be compared against each other. Generally, there is an improvement in the AlN encapsulated material quality up to 1400° C., but the results degrade for annealing temperatures >1400° C. This degradation is due to reliability issues associated with the AlN cap at higher temperatures, and possibly because the heating rates (50° C./s) are still not high enough to prevent GaN decomposition. However, with the much higher heating rates achievable with the microwave annealing system used in this embodiment, there is the possibility of reliably annealing GaN at temperatures higher than 1400° C.
B. Microwave Annealing of In-Situ Mg Doped GaN
1. Experimental Details
The samples explored in this experiment were 3 μm thick Mg-doped GaN epilayers on a-plane sapphire substrates grown by metalorganic chemical vapor deposition (MOCVD). To heat the GaN sample, a 5 mm×5 mm highly conducting 4H—SiC piece is placed directly underneath the GaN sample of interest to serve as the susceptor, when both the GaN sample and the SiC piece are placed within the microwave heating head. Microwave annealing of GaN is performed with and without a surface capping layer composed of MgO, AlN or graphite. The AlN layers (200 nm thick) were deposited on the GaN sample using pulsed-laser deposition. The MgO layers (200 nm thick) were deposited on the GaN using electron beam evaporation of a MgO target. Fused lumps of MgO (Alfa Aesar, 99.95%, metals basics, 3-12 mm pieces) were used as target material. Graphite caps are formed on the GaN epilayers by first spin-coating a layer of standard photoresist, followed by annealing in vacuum at 750° C. Microwave annealing is performed in the temperature range of 1300-1350° C. for short 5 s durations in a pure (99.999%) nitrogen atmosphere. After microwave annealing, the MgO cap is removed by etching in dilute acetic acid, whereas the AlN cap is removed by a 10 min etch in 85 wt% H₃PO₄at 80° C.
The reliable application and removal of the AlN cap on the GaN surface was studied using XPS. The XPS spectra were acquired using a Mg Kα x-ray source. The sample surface after annealing and removal of the cap is monitored by tapping mode AFM. The optical characterization of the material is performed using low-temperature (5 K) photoluminescence (PL) spectroscopy. For obtaining the PL spectra, a He—Cd laser was used with an excitation intensity of 2.5 mW. Details about the PL system are generally known. Room temperature Hall measurements were performed after depositing (30 nm) Ni/(30 nm) Au contacts on the GaN layers in the van der Pauw geometry. The contacts were made ohmic by alloying in a conventional box furnace at 350° C., in air, for 10 min.
2. XPS Characterization of AlN Capped GaN
The reliability of the application, sustainability of the AlN cap during annealing, and removal of the AlN cap after microwave annealing was studied by XPS. Referring to FIG. 5, part (a) shows a survey XPS scan of the surface of the AlN as-capped sample. Other than O 1s and C 1s signals coming from the native oxide/hydrocarbon layer, only Al and N signals can be seen in the survey scan. Part (b) shows the survey XPS scan of the AlN capped sample after 1400° C./5 s microwave annealing. Surprisingly, no nitrogen signal can be detected from this scan, but a rather strong O 1s signal is seen in addition to the Al signal. Narrow scans (not shown) of the O 1s peak were consistent with the presence of either Al₂O₃or Al(OH)₃.
Thus, upon microwave annealing, the AlN film has oxidized and formed Al₂O₃and/or Al(OH)₃. This result occurred despite annealing in a 99.999% atmosphere of UHP nitrogen, which emphasizes the strong oxidation affinity of the AlN film. A similar result was obtained after microwave annealing at 1500° C./5 s. Part (c) of FIG. 5 shows a survey XPS scan of the sample after 1400° C. microwave annealing and removal of the cap by H₃PO₄. Clearly, Ga and N signals can be observed for this XPS scan, but no Al signals are observed indicating that the AlN cap was successfully removed. Again, a similar XPS scan was obtained for the 1500° C. annealed sample, as well as after the AlN cap removal.
3. Surface Morphology of the Microwave Annealed Samples
Tapping mode AFM images may be taken for a) an as-grown GaN surface (RMS=0.3 nm); after 1300° C./5 s microwave annealing of GaN layers with (b) no cap (RMS=9.2 nm), (c) MgO cap (RMS=0.8 nm), and (d) AlN cap (RMS=1 nm); (e) after 1400° C./5 s annealing with MgO cap (RMS=7.2 nm); and (f) after 1500° C./5 s annealing with AlN cap (RMS=0.6 nm). In short, these images are based on the GaN sample surface, after microwave annealing at different temperatures with a MgO cap in place. The MgO cap was able to protect the GaN surface without any substantial decomposition at annealing temperatures up to 1300° C., but significant GaN decomposition could be detected for the MgO capped annealing done at 1400° C. (part (e)). The GaN film totally decomposed, when the microwave annealing temperature was increased above 1400° C. Decomposition of the GaN was accompanied by cracking of the MgO cap, and liquid Ga droplets could be observed (not shown) on the surface.
For (d) and (f), microwave annealing took place at 1300° C. and 1500° C., respectively, where both had an AlN cap in place. In (f), the GaN surface of the 1500° C./5 s microwave annealed sample, with an AlN cap in place appears very smooth with a RMS roughness (0.6 nm) comparable to the as-grown sample (0.3 nm). No evidence of any GaN decomposition can be seen for even this ultra-high-temperature AlN capped annealing. For comparison, (b) refers to an AFM image of a GaN sample annealed for 5 s at 1300° C. without any cap in place. Significant GaN decomposition resulting in the formation of hexagonal cavities can be observed in (b).
To summarize, ultra-fast microwave annealing was successfully used to anneal GaN epilayers up to temperatures as high as 1500° C. using a protective pulsed laser deposition (PLD) AlN capping layer. The PLD deposited AlN film is a much better capping layer to preserve the surface integrity of GaN at temperatures >1300° C. compared to the e-beam deposited MgO film. It might seem that the MgO film might have cracked due to a greater lattice mismatch between the GaN and MgO (˜6.5%) compared to the GaN and AlN (2.6%). However, x-ray diffraction scans (not shown) confirmed that the e-beam deposited MgO layer is fine-grain polycrystalline. Thus, the MgO layer should have plenty of grain boundaries to accommodate lattice or thermal co-efficient of expansion (TCE) mismatch without cracking. In fact, significant GaN decomposition was observed for the MgO capped sample annealed at 1350° C., 50° C. before the MgO film cracked, whereas the AlN capped samples remained decomposition-free even after a 1500° C. treatment. It is known that the PLD process used to deposit the AlN cap results in a much better interface with the underlying GaN compared to the e-beam deposition process, which was used for the MgO cap formation. Thus, the presence of a large number of voids at the MgO/GaN interface could have allowed the escape of nitrogen from the GaN film, which accelerated the decomposition of the GaN film. It would be interesting to explore pulsed laser deposited MgO caps for protecting the GaN surface during high-temperature microwave annealing.
In addition to the MgO and AlN caps, photoresist converted graphite caps were also explored to study their feasibility for protecting the GaN surface during high-temperature microwave annealing. Current literature has shown that graphite caps have been successfully protected SiC epilayers during ultra-high temperature (1700-1900° C.) microwave annealing of SiC. However, the present experiments have found that for microwave annealing of GaN, the graphite caps started delaminating from the GaN surface at temperatures >1000° C., presumably because of the stress at the GaN/graphite interface, created by localized decomposition of the GaN epilayer under the graphite cap. From this study, it is evident that an excellent interface between the GaN and the capping layer is vital, if the GaN surface morphology is to be preserved during high-temperature annealing.
4. Photoluminescence Characterization
FIG. 6 shows low-temperature (5 K) PL spectra on the in-situ Mg-doped GaN films annealed at 1300° C. and 1500° C. for a duration of 5 s, using AlN cap layer. The PL spectrum from an as-grown (unannealed) sample is also shown for comparison. The only feature visible in the PL spectra from the as-grown sample is a broad band (3.0-3.2 eV), with no phonon replicas. This band may be a superposition of at-least two components, the blue luminescence (BL) band and the ultraviolet luminescence (UVL) band. Known to appear at 2.9-3.1 eV in heavily Mg-doped GaN grown by MOCVD, the BL band is supposed to be due to photo-excited carriers from a deep localized donor recombining with the shallow Mg acceptor. Appearing at ˜3.1-˜3.2 eV in heavily Mg-doped and compensated GaN, the UVL band is supposed to originate from the donor-acceptor pair (DAP) recombination between a shallow donor level (presumably O_N) and the Mg acceptor. The UVL band appears broad and featureless for the as-grown sample. This appearance is possibly due to potential fluctuations arising from the random distribution of charged impurities such as donors and acceptors, coupled with the fact that there are not enough free carriers to screen them. For the sample annealed at 1300° C., the relative intensity of the blue band reduces. The UVL band increases in intensity and is significantly blue shifted to yield a zero-phonon line (ZPL) at 3.27 eV along with its two LO phonon replicas at 3.18 eV and 3.09 eV. Also, a near band-edge emission peak corresponding to the recombination of an exciton bound to a neutral donor (D° X) can also be observed from FIG. 6. A decrease in the intensity of the blue band and the appearance of the D° X band indicate that the concentration of the compensating deep donors has reduced due to the microwave annealing, thus activating the Mg acceptors. This activation can be seen by the blue shift as well as the increase in both intensity and structure of the UVL band. For the sample annealed at 1500° C., the relative intensity of the blue band decreases further, whereas the intensities of the UVL band and the near-band-edge emission band increase. This indicates that the 1500° C./5 s microwave annealing is more effective than the 1300° C./5 s annealing in reducing the concentration of the compensating deep donor levels and, therefore, in activating the Mg dopants.
Referring to FIG. 7, low-temperature PL spectra, on the microwave annealed samples with a MgO cap in place, indicate an increase in Mg activation for 1300° C./5 s annealed sample by the presence of an intense, structured DAP UVL band at 3.29 eV (ZPL) as well as a strong near-band-edge emission (D° X) band at 3.46 eV. However, upon increasing the annealing temperature to 1350° C., the D° X band disappears, whereas a broad blue band (2.7-3.1 eV) is observed, which is red shifted even more than the 3.0-3.2 eV band observed in the spectra of the as-grown sample. Since the AFM images did show a significant increase in GaN decomposition for the 1350° C. annealing, it is conceivable that this broad band originates from a number of deep donor-like defects (such as V_N), which were created by the decomposition. A similar spectra (not shown) was also obtained for the 1400° C./5 s annealed sample, with the MgO cap in place.
The above PL results suggest that high-temperature (1500° C.) microwave annealing using AlN cap is very effective in increasing the net acceptor concentration by decreasing the concentration of the compensating deep donors in GaN.
5. Electrical Characterization
FIG. 8 shows a variation of the hole concentration (p) as a function of microwave annealing temperature, for the uncapped samples and for samples protected by the MgO and AlN caps during 5 s microwave annealing. For both uncapped as well as MgO capped samples, the p decreases, when the annealing temperature is increased above 1300° C. This decrease is a direct result of increasing GaN decomposition with increasing annealing temperature for uncapped and MgO capped GaN layers, which was observed from the AFM images. The PL spectra for the MgO capped samples also indicated a decrease in Mg acceptor activation for the 1350° C. and 1400° C. anneals compared to the 1300° C. annealing, which agrees with the electrical results.
However, for the samples which were capped by the AlN during annealing, the highest p is measured for the 1500° C. annealing. Based on the above PL results, this phenomenon is likely due to a decrease in the compensating deep donor concentration with increasing annealing temperature as long as the integrity of the GaN material is maintained. Relatively high hole mobilities of 14-19 cm2/V.s were measured on all the above-mentioned samples. No change in the hole mobility after the microwave annealing treatment was observed.
6. Summary of Microwave Annealing of In-Situ Mg Doped GaN
Because of the ultra-fast heating/cooling rates of the microwave RTA system, the GaN can be successfully annealed in the temperature range of 1300-1500° C., when the GaN is protected by a pulsed laser deposited AlN cap. The surface of the AlN capped GaN layer annealed at 1500° C. for 5 s is very smooth with a RMS roughness of 0.6 nm, which is comparable to the RMS roughness of 0.3 nm measured on the as-grown sample. The e-beam deposited MgO cap successfully protected the GaN surface during microwave annealing only up to 1300° C., but a significant GaN decomposition is observed for the higher temperature anneals. Low-temperature (5 K) PL spectra and Hall measurements performed on the AlN capped samples indicate that the 1500° C./5 s microwave annealing is more effective than the 1300° C./5 s microwave annealing in activating the Mg-dopant by decreasing the concentration of the compensating deep donor levels present in the as-grown sample. By comparison, fairly good luminescence and electrical results were obtained for the e-beam deposited MgO capped GaN layers only for annealing at 1300° C., but the optical and electrical quality of the GaN layers degrade during higher-temperature (>1300° C.) annealing. Photoresist converted graphite cap delaminates from the GaN surface for microwave annealing temperatures >1000° C. and is therefore not a suitable capping material for high-temperature annealing of GaN.
C. Microwave Annealing of Mg-Implanted GaN
After demonstrating improvement in the optical and electrical properties of in-situ Mg-doped GaN after high-temperature (1300-1500° C.) microwave annealing, the logical next step was to explore the feasibility of microwave annealing on Mg ion-implanted GaN. The Mg-implanted GaN layers could be used as the base region in a GaN heterojunction bipolar transistor (HBT). Also, selective Mg-implants through an implantation mask could be used to more easily create arrays of GaN LEDs and laser diodes as opposed to reactive ion etching p-type GaN epilayers.

1. EXAMPLE 1

a. Implantation and Annealing Schedules
TABLE 3 shows the multiple energy Mg⁺implant schedule performed into undoped 3 μm GaN epilayers grown on a-plane sapphire.

TABLE 3

Multiple Energy Mg Implant Schedule
Performed into Undoped GaN

	Implant Energy (keV)	Implant Dose (cm⁻²)

	10	3.8 × 10¹³
	25	3.3 × 10¹⁴
	55	1.7 × 10¹⁴
	110	4.1 × 10¹⁴
	225	8.3 × 10¹⁴
	300	8.3 × 10¹⁴

The implantation was performed at a temperature of 500° C. with a tilt of 7°. As in case of SiC, the multiple energy Mg implant schedule for GaN was also designed using the SRIM-2006 software. FIG. 9 shows a comparison of the simulated and the experimental (SIMS) Mg implant profiles. It can be observed that there is a significant discrepancy between the simulated and the experimentally determined Mg implant profiles. The simulation predicts a higher Mg concentration and a smaller ion penetration depth, whereas the experimentally measured profile displays a longer implant tail into the substrate. A similar discrepancy between simulated and experimental Si implant profiles in GaN was observed in previous literature. Thus, some work will need to be done to obtain better stopping powers for implanted ions in GaN.
After implantation, the GaN epilayers were capped by a 0.3 μm layer of AlN grown by PLD and then subjected to microwave annealing in the range of 1300-1500° C. After annealing, the AlN cap was etched by the H₃PO₄recipe, as described earlier. The reliable removal of the AlN caps after microwave annealing was again confirmed by the XPS measurements. After removing the cap, the Mg-implanted GaN epilayers were characterized for their structural and electrical properties, and also for the thermal stability of the implant.
b. SIMS Depth Profiling
FIG. 10 shows SIMS depth profiles of the Mg implanted GaN before and after 1300° C./5 s and 1400° C./5 s microwave annealing. The SIMS profile for the as-implanted sample and for the 1300° C./5 s annealed sample are close. However, a slight Mg accumulation at the surface and some in-diffusion of Mg into the GaN can be observed for the 1300° C. annealed sample. The microwave annealing at 1400° C. resulted in a significant Mg accumulation in a thin ≈40 nm surface layer, and a depletion of Mg at depths of 40 nm-400 nm from the surface. A pronounced in-diffusion of Mg into the GaN can also be observed at depths beyond 400 nm. As illustrated, the extracted doses from the 1300° C. (1.6×10¹⁵cm⁻²) and 1400° C. (1.5×10¹⁵cm⁻²) annealed samples are slightly lower compared to the extracted dose (1.7×10¹⁵cm⁻²) from the as-implanted sample. This difference is probably due to some out-diffusion of Mg into the AlN cap during the annealing treatment.
c. Photoluminescence Characterization
FIG. 11 shows low-temperature PL spectra from Mg-implanted GaN, before and after 1400° C./5 s and 1500° C./5 s microwave annealing. For reference, the PL spectra from an as-grown GaN epilayer used for the Mg-implantation is also shown. In addition to the near-band edge emission, a broad yellow luminescence (YL) band (2.0 eV-2.6 eV) and a broad blue luminescence (BL) band (2.7 eV-3.2 eV) also can be observed in the PL spectra obtained from the as-grown GaN epilayer. As discussed earlier, the appearance of BL in low-temperature PL spectra of GaN is attributed to the presence of donor-like states in the bandgap, which may arise from V_Nor pyramidal defects. The YL in GaN is generally attributed to the presence of C, O, and H in the material. The presence of YL and BL in the PL spectra indicates a poor quality GaN material, especially for p-type doping, since the YL and BL can severely compensate the activated acceptors.
The as-implanted GaN does not exhibit any photoluminescence, since the implant-induced damage introduces a lot of defect levels in the bandgap, which act as non-radiative recombination centers. The PL spectra from the 1400° C. microwave annealed GaN does show the re-appearance of the near band-edge D° X emission as well as DAP emission related to Mg activation. Thus, microwave annealing at 1400° C. at-least partially heals the implant-induced lattice damage. Increasing the annealing temperature to 1500° C. results in further recovery of implant-induced damage as can be seen from the increase in intensity of both D° X emission and Mg activation related DAP emission from the PL spectra of FIG. 11. However, the YL and BL bands can also be seen in the PL spectra of microwave annealed samples, which possibly precludes any electrical activation due to compensation of the activated acceptors. Thus, it is paramount to have an excellent quality GaN epilayer which doesn't emit YL and BL, especially for fabricating device structures which require p-type implantation.
d. Electrical Characterization
Electrical characterization of the GaN even after a 1500° C. annealing treatment has indicated almost no electrical activation of Mg. The samples are highly resistive. This result is likely due to the significant lattice damage created by the high dose, multiple energy Mg implant. Also, as shown in FIG. 11, the PL spectra have indicated the presence of compensating deep levels even in the as-grown GaN epilayer.

2. EXAMPLE 2

a. Experimental Details
Single-energy (150 KeV) Mg implantation was performed into unintentionally n-type doped, 3 μm thick GaN films grown on c-plane sapphire using hydride vapor phase epitaxy (HVPE). An implantation dose of 5×10¹⁴cm⁻²was applied on the wafer tilted 7° and kept at 500° C. Multiple-energy Mg ion-implantation was also performed at 500° C. in some samples, using the schedule shown in TABLE 3. The implanted surface of the GaN samples was then capped with 6000 Å thick AlN layer using PLD to protect the surface of the GaN films during the high temperature annealing process.
Microwave annealing was performed on implanted GaN films using a highly conducting 4H—SiC piece placed directly underneath the sample to serve as a susceptor. During the annealing, the implanted surface of GaN/Sapphire, capped with thin AlN film, was placed with the GaN facing down on a polished surface of the 4H—SiC susceptor. It is important to mention that the SiC susceptor is not required if the GaN layer is grown on a conducting hexagonal SiC substrate. However, for extra protection of GaN surface, it is advisable to place the AlN capped GaN layer in intimate contact with another polished SiC or sapphire piece. Both the sample and the susceptor were placed in the microwave heating head and the temperature was measured using an optical pyrometer. Microwave annealing was performed in a temperature range of 1300-1600° C. for 5-15 s in pure (99.9%) nitrogen atmosphere. After annealing, the AlN cap was removed by a 1 hour etch in 85% wt H₃PO₄at 120° C. The removal of the AlN cap from the GaN surface was confirmed by XPS.
The sample surface is monitored, after annealing and removal of the cap, by tapping mode AFM. High-resolution X-ray diffraction measurements were taken using an 18 kW Rigaku ATX-E diffractometer with Cu radiation. Two channel cut Ge (220) crystals were used to monochromatize the incident beam and provide parallel beam of CuKα₁radiation. This arrangement almost eliminates the vertical divergence in the incident beam and provides a precise measurement of the rocking curve and lattice parameters. In addition, this diffractometer is equipped with an open Eulerian cradle with independent (x,y,z) movements and the tilt, χ and rotation, φ movements which are necessary for precise alignment of the sample. The optical characterization of the as-grown, implanted, and annealed samples was performed using low-temperature (5 K) photoluminescence spectroscopy. The luminescence was excited with the 325 nm line of a He—Cd laser with an excitation intensity of 2.5 mW. The light emiited by the samples was dispersed by a high-resolution spectrometer fiited with a UV-sensitive photomultiplier coupled to a photon counter. SIMS measurements were also performed on the as-doped and annealed samples to check the thermal stability of the dopants.
b. AFM and SIMS Configuration
Tapping mode AFM images may be taken of single energy Mg-implanted GaN sample surface, after 1300° C./15 s, and 1500° C./15 s microwave annealing. The rms roughness before annealing is 0.3 nm. After 1300° C./15 s annealing, it is 1.5 nm. The rms surface roughness increased with increasing annealing temperature. The AlN cap provided a reasonable surface protection for GaN even for 1500° C./15 s annealing, yielding an rms roughness of 6 nm. The results shown are for the samples in which the AlN capped (600 nm thick) implanted surface was placed face down on the polished surface of the 6H—SiC susceptor. For samples with smaller AlN cap thickness and where the AlN capped surface is not placed in intimate contact with the SiC susceptor, an rms surface roughness of 50 nm or more was observed after 1500° C./15 s annealing. Based on these results, even if the GaN film is grown on a conductive hexagonal SiC substrate, it is advisable to place it face down on a polished SiC substrate to preserve the surface morphology.
Referring to FIG. 10 again, Mg implant SIMS depth profiles may be recorded on as-implanted and microwave annealed (1300° C./5 s and 1400° C./5 s) multiple energy Mg-implanted samples. The SIMS profile for the as-implanted sample and for the 1300° C./5 s annealed sample are similar. However, a slight Mg accumulation at the surface and some in-diffusion of Mg into the GaN film (toward the GaN-substrate interface) can be observed for the 1300° C. annealed sample. The microwave annealing at 1400° C. resulted in a significant Mg accumulation in a thin ≈40 nm surface layer, and a depletion of Mg at depths of 40 nm-400 nm from the surface. A pronounced in-diffusion of Mg into the GaN can also be observed at depths beyond 400 nm. The extracted doses from the 1300° C. (1.6×10¹⁵cm⁻²) and 1400° C. (1.5×10¹⁵cm⁻²) annealed samples are slightly lower compared to the extracted dose (1.7×10¹⁵cm⁻²) of the as-implanted sample. This finding may result from some out-diffusion of Mg into the AlN cap during the annealing treatment.
c. X-Ray Characterization
FIG. 12 shows HRXRD spectra of the GaN (004) reflections of the as-implanted and microwave annealed multiple energy Mg-implanted samples. It can be seen from the HRXRD spectra that the sub-lattice defect peak, which is due to the interference of the X-rays from the implanted impurity in the interstitial sites, did not disappear after annealing. This peak results from the excessive damage produced by the multiple energy implantation process in the GaN films, which could not be eradicated by the microwave annealing process. The lattice parameters and rocking curve full width at half maximum (FWHM) values for the multiple-energy Mg-implanted GaN films show no significant improvement upon annealing of these samples. Because the multiple-energy implantation process caused severe damage in the samples, single-energy Mg ion-implantation was performed for further study of Mg-acceptor activation in GaN.
FIG. 13 shows an overlay of the XRD scans of the GaN (004) reflections of the as-grown, as-implanted, and microwave annealed single-energy Mg-implanted GaN samples. It can be seen that the implanted sample has the defect sub-lattice peak as shown earlier in FIG. 12 for the multiple energy Mg-implantation. For single-energy Mg-implantation, the defect sub-lattice peak disappears after microwave annealing (unlike in multiple energy Mg-implanted samples), confirming that the implanted Mg has moved to electrically and optically active substitutional lattice positions. In addition, the FWHM values, shown in TABLE 4, also decrease upon annealing, indicating that the implant induced damage has been removed. Upon 1300° C./15 s annealing, the FWHM value decreases to the un-implanted sample level. For higher temperature anneals, the FWHM value goes down further and reaches towards values lower than that of the un-implanted samples. Consistent with previously reported results for ion-implanted SiC, this observation suggests that the high temperature microwave annealing process may also remove some of the growth related defects in the un-implanted region of GaN films, improving their crystalline quality. This effect is possible, because the GaN films growth temperatures are below the microwave annealing temperatures.

TABLE 4

a and c Lattice Parameters and Rocking Curve FWHM
Values of the Virgin, As-implanted, and 15 s Microwave
Annealed Single-Energy Mg-implanted GaN Films

			FWHM		C, A
Sample	Reflection	Θ	(arcsecs)	2Θ	(Å)

Virgin	(004)	36.745	293	72.900	5.1860
	(104)	16.918	209	82.040	3.1890
as implanted	(004)	36.819	310	72.900	5.1860
	(104)	16.741	231	82.027	3.1915
1300° C.	(004)	36.132	273	72.908	5.1855
	(104)	15.345	197	82.036	3.1914
1400° C.	(004)	36.098	248	72.907	5.1856
	(104)	15.508	206	82.035	3.1913
1500° C.	(004)	36.732	252	72.906	5.1857
	(104)	16.879	207	82.038	3.1906

d. PL Measurements
FIG. 14 shows low-temperature PL spectra from multiple-energy Mg-implanted GaN samples, before and after 1400° C./5 s and 1500° C./5 s microwave annealing. Also seen in the PL spectra obtained from the as-grown GaN epilayer is the near-band edge (NBE) emission, a broad yellow luminescence (YL) band (2.0 eV-2.6 eV), and a near-ultra violet luminescence (N-UVL) band (2.7 eV-3.2 eV). The YL band in GaN is generally associated to the presence of C, O, and/or H in the material. The YL and N-UVL bands in the PL spectra indicate the presence of deep levels in un-intentionally doped (UID) as-grown GaN films.
The as-implanted GaN sample does not exhibit any significant photoluminescence in the probed spectral range. Implantation-induced damage introduces a number of defects in the bandgap. Such defects act as non-radiative recombination centers or radiative centers emitting in different spectral range. The PL spectrum of the 1400° C. microwave annealed sample show a weak NBE emission, N-UVL bands (2.7 eV-3.2 eV), and a relatively intense YL band. Thus, microwave annealing at 1400° C. at least partially heals the implantation-induced lattice damage. The PL spectrum of the 1500° C. annealed sample shows a relatively large increase of the NBE and N-UVL band intensities. Meanwhile, no intensity variation is observed for the YL band. In addition, it can be clearly observed that at ˜3.26 eV, the no-phonon line of the recombination process associated with shallow DAP. Therefore, increasing the annealing temperature to 1500° C. results in further reduction of the implantion-induced damage. However, the defects related to the YL and N-UVL bands are still the dominant recombination channels as compared with those involving NBE and DAP emission bands, which are consistent with the small activation of the Mg-impurities and persistent lattice damage.
FIG. 15 shows low-temperature PL spectra of the single-energy Mg-implanted GaN annealed at 1300° C., 1400° C., 1500° C. (for 15 s), and the as-implanted (un-annealed) samples. Similar to the PL spectrum of the un-annealed multiple-energy Mg-implantation shown in FIG. 14, the spectrum of the un-annealed single-energy Mg-implantation, represented in FIG. 15, shows an overall decrease of the luminescence intensity. However, the NBE luminescence emission of the latter did not evanesce likewise that of the former. This result is consistent with the XRD results, indicating lower ion-implantation lattice damage. Similar to the multi-energy implanted sample set, the intensity of the YL reach its maximum at 1300° C./15 s, showing no strong dependence with the annealing temperature, while the N-UVL intensity increases with increasing annealing temperature. The no-phonon line of the DAP band is clearly observed in the spectra of the samples annealed at 1400° C. and 1500° C., overlapping the N-UVL band. It is difficult to identify the phonon replicas of the DAP due to the interference fringes modulating the spectra. An emission band between 3.27 eV and 3.37 eV is observed in the spectra of the sample annealed at 1300° C., and may be weakly observed in the 1400° C. annealed sample spectrum. But, it is completely quenched in the 1500° C. annealed sample spectrum. This band may be associated with a structural defect with low thermal annealing temperature. Additional experimentation is necessary to determine its nature. It should be noted that despite the increase of the N-UVL and DAP band intensities, the intensity of the NBE emission band did not increase. This lack of increase may be consistent with increasing concentration of the Mg-acceptor, along with increasing annealing temperature, which compensate the shallow donor(s) associated with the NBE emission band.
e. Electrical Configuration
The electrical characterization of the multiple energy Mg-implanted GaN films, even after a 1500° C. annealing treatment, has indicated almost no net electrical activation of the Mg-acceptor implant. The samples remained highly resistive after annealing and are likely due to the high degree of lattice damage created by the high-dose, multiple-energy Mg-implantation. Though the small DAP related band was observed in the PL spectra of the annealed samples, the x-ray measurements indicated a high degree of residual implant damage in the annealed samples. Also, the XRD spectra (not shown) and PL spectra (FIG. 14) indicated the presence of defects even in the as-grown GaN films used for the multiple energy implantation, which may influence the Mg activation. The combination of the poor starting material and high residual implant damage remaining in the material even after 1500° C. annealing are detrimental for the electrical activation of the multiple-energy Mg implant. No electrical activation was observed even after 15 s annealing at 1500° C.
FIG. 16 shows two-probe current-voltage (I-V) measurements on the single-energy Mg implanted GaN films and acceptor activation in the microwave annealed samples when the samples were annealed for 15 s. No net electrical activation was observed for 5 s annealing. The breakdown voltage decreased with increasing annealing temperature, for 15 s annealing, indicating an increase in the Mg-acceptor activation with increasing annealing temperature. The extrapolated resistivity from the linear segments of the I-V curves is in the range of 0.4 to 1.4 Ω-cm for annealing temperatures of 1500° C. to 1300° C. Hence, based on the XRD, PL and I-V measurements it can be stated that the single energy Mg implanted films, after 15 s high temperature microwave annealing have shown improved crystalline quality and activation of the Mg-implant. The net Mg acceptor activation stems from an increase in Mg substitutional acceptor activation coupled with an effective decrease in the density of compensating background donor defects (like residual nitrogen vacancy introduced by implantation).
D. Summary and Future Work on Microwave Annealing of GaN
For Example 1, GaN epilayers were reliably annealed at high-temperatures in the range of 1300-1500° C., when the GaN is protected by a PLD AlN cap. Promising electrical and optical results were obtained for in-situ Mg doped epilayers. However, it has proven to be more challenging to activate a multiple energy, high dose Mg implanted GaN. Significant lattice damage exists even after annealing at temperatures as high as 1500° C., albeit for short 5 s durations. Lower dose single-energy Mg implantations are planned on GaN epilayers, which are of a much higher quality than the ones explored in the present study.
For Example 2, a novel ultra-fast microwave annealing method, different from conventional thermal annealing, is used to successfully activate Mg-implants in GaN layer. The x-ray diffraction measurements indicated complete disappearance of the defect sub-lattice peak, introduced by the implantation process for single-energy Mg-implantation, when the annealing was performed at ≧1400° C. for 15 s. An increase of the intensity of Mg-acceptor related luminescence peak (at 3.26 eV) in the photoluminescence spectra combined with a decreasing Schottky breakdown voltage confirm the net Mg-acceptor activation of single-energy Mg-implanted GaN. In the case of multiple-energy implantation, the implant generated defects persisted even after 1500° C./15 s annealing, resulting in no net Mg-acceptor activation of the Mg-implant. The Mg-implant is relatively thermally stable and the sample surface roughness is 6 nm after 1500° C./15 s annealing, using a 600 nm thick AlN cap.
It is found that p-type activation can be achieved by microwave annealing of single-energy Mg-implanted GaN for annealing temperatures equal to or above 1400° C. for 15 s. The XRD and PL results and the two-probe current-voltage measurements confirm the effective removal of lattice damage and Mg acceptor activation in single-energy Mg-implanted GaN films. For multiple-energy Mg-implantation, we could not observe net p-type conduction even for 15 s annealing due to a large degree of implant lattice damage.
Future work involves ultra-high temperature annealing of Si (n-type) implanted GaN and especially AlGaN epilayers grown on SiC. If successful, such layers can be used under source/drain metal contacts of AlGaN—GaN HEMT devices, in an attempt to lower the source/drain access resistance, and to increase the device transconductance. Also, future high-temperature microwave anneals are planned on in-situ Mg doped Al_0.25Ga_0.75N and Al_0.4Ga_0.6N grown on sapphire. Increasing the Al content in the AlGaN ternary increases the bandgap and finds application in smaller wavelength laser diodes. However, the increasing Al content in AlGaN also makes p-type doping more difficult to achieve.

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VI. Statements
While various embodiments have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art(s) that various changes in form and detail can be made therein without departing from the spirit and scope. In fact, after reading the above description, it will be apparent to one skilled in the relevant art(s) how to implement alternative embodiments. Thus, the present embodiments should not be limited by any of the above described exemplary embodiments.
In addition, it should be understood that any figures which highlight the functionality and advantages, are presented for example purposes only. The disclosed architecture is sufficiently flexible and configurable, such that it may be utilized in ways other than that shown. For example, the steps listed in any flowchart may be re-ordered or only optionally used in some embodiments.
Further, the purpose of the Abstract of the Disclosure is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract of the Disclosure is not intended to be limiting as to the scope in any way.
Finally, it is the applicant's intent that only claims that include the express language “means for” or “step for” be interpreted under 35 U.S.C. 112, paragraph 6. Claims that do not expressly include the phrase “means for” or “step for” are not to be interpreted under 35 U.S.C. 112, paragraph 6.

Claims

1. A doped gallium nitride (GaN) annealing method comprising:

a. placing, within a heating unit, a silicon carbide (SiC) wafer as a susceptor in close proximity with a doped GaN epilayer, wherein the doped GaN epilayer is either a GaN layer grown on a substrate or a GaN layer that is free standing; and

b. heating, at a heating rate of at least about 100° C./s, the wafer and the doped GaN epilayer to at least about 1200° C.

2. The method according to claim 1, further including depositing a protective capping layer on the doped GaN epilayer.

3. The method according to claim 2, wherein the protective capping layer is aluminum nitride (AlN).

4. The method according to claim 3, wherein the AlN is deposited using pulsed-laser deposition.

5. The method according to claim 2, wherein the protective capping layer is magnesium oxide (MgO).

6. The method according to claim 1, wherein the method is performed in a vacuum chamber or an inert gas atmosphere.

7. The method according to claim 1, wherein the doped GaN is either in situ doped or ion-implantation doped.

8. The method according to claim 1, wherein a dopant used for the doped GaN is:

magnesium, beryllium, calcium, zinc, silicon, sulfur, iron, cobalt, vanadium, or any combination thereof.

9. The method according to claim 1, wherein the heating unit is a microwave heating head.

10. The method according to claim 1, wherein the heating unit is a laser annealing system.

11. A doped gallium nitride (GaN) annealing method comprising:

a. placing, within a heating unit, a doped GaN epilayer, the doped GaN epilayer being either a GaN layer grown on a conducting substrate or a GaN layer that is free standing; and

b. heating, at a heating rate of at least about 100° C./s, the doped GaN epilayer to at least about 1200° C.

12. The method according to claim 11, further including depositing a protective capping layer on the doped GaN epilayer.

13. The method according to claim 12, wherein the protective capping layer is aluminum nitride (AlN).

14. The method according to claim 13, wherein the AlN is deposited using pulsed-laser deposition.

15. The method according to claim 12, wherein the protective capping layer is magnesium oxide.

16. The method according to claim 11, wherein the doped GaN is either in situ doped or ion-implantation doped.

17. The method according to claim 11, wherein a dopant used for the doped GaN is:

18. The method according to claim 11, wherein the method is performed in a vacuum chamber or an inert gas atmosphere.

19. The method according to claim 11, wherein the heating unit is a microwave heating head.

20. The method according to claim 11, wherein the heating unit is a laser annealing system.