WO2000065666A1

WO2000065666A1 - Visible light emitting device formed from wide band gap semiconductor doped with a rare earth element

Info

Publication number: WO2000065666A1
Application number: PCT/US2000/010283
Authority: WO
Inventors: Ronald H. Birkhahn; Liang-Chiun Chao; Michael J. Garter; Andrew J. Steckl
Original assignee: University Of Cincinnati
Priority date: 1999-04-23
Filing date: 2000-04-17
Publication date: 2000-11-02
Also published as: EP1196954A4; CA2371361A1; EP1196954A1; US6406930B2; AU4355100A; US6255669B1; KR20020025863A; JP2002543595A; US20010015469A1

Abstract

A visible light emitting device includes a wide band gap semiconductor layer doped with one or more elements which emit light at various wavelengths based upon atomic transitions. The semiconductor preferably is GaN, InN, AIN, BN or alloys thereof doped with a lanthanide element such as Er, Pr or Tm. The light emission can be enhanced by annealing the WBGS.

Description

Visible Light Emitting Device Formed From Wide Band Gap Semiconductor Doped With a Rare Earth Element

Background of the Invention

Light emitting diodes (LED) and related light emitting devices are used in a vast number of applications. These can be used in most light emitting devices from simple panel lights to complex displays and lasers. Currently LEDs are used in the automotive industry, consumer instrumentation electronics, and many military applications. Different compounds are used to produce different wavelengths of light. For example, aluminum gailium arsenide is used for red LEDs, gallium aluminum phosphide for green, and GaN for blue. Light emitting

materials formed from three different materials are often difficult to produce. Utilizing different LEDs together inherently requires allowing for different performance characteristics such as current and voltage requirements.

Wide band gap semiconductors (WBGS) doped with light emitting elemenis such as rare earth elements (RE) and other elements with partially filled inner shells are particularly attractive for LEDs because the emission efficiency appears to increase with band gap value, thus allowing room temperature operation without the need to introduce impurities. Wide band gap generally refers to a band gap of 2 eV or greater. Electroluminescence has been reported from several WBGS hosts including Er-doped gallium arsenide, gallium phosphide, GaN, ZnSe and SiC. Er-doped semiconductor light emitting diodes have been shown to emit in the infrared at about 1.5 microns. The infrared emission corresponds to transmissions between the lowest excited state (⁴l_13/2) and the ground state (⁴l_15/2) of the erbium atoms. The first Er-doped semiconductor light emitting diodes emitted IR light only at very low temperatures. However, recent advancements have permitted IR light emission at near room temperature. Although IR emitting Er-doped GaN has a great deal of utility in the communications industry, it previously has not been useful in a light emitting diode requiring visible emission. Summary of the Invention

The present invention is premised on the realization that wide band

gap semiconductor substrates doped with elements with partially filled inner shells such as rare earth elements and transition metals can be formed and will emit in the visible and ultraviolet spectrum at a wide range of temperatures. The wide band gap semiconductor material are group lll-V and IV materials including diamond, GaN, AIN, InN, BN and alloys

thereof. These are doped with elements such as cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, turbium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetium or other elements with partially filled inner shells. By proper formation of the wide band gap semiconductor material and proper introduction of the rare earth element, a light emitting diode can be formed which emits in the visible spectrum

By selection of the appropriate dopant material, one can select the appropriate color For example, in GaN, erbium will produce green whereas thulium will produce blue and praseodymium will produce red.

The objects and advantages of the present invention will be further appreciated in the lignt of the following detailed description and drawing in which

Brief Description of the Drawings

The Figure is a graph depicting the PL spectrum of Pr-impianted GaN films treated under different annealing conditions.

Detailed Description

In order to form a light emitting devices accorαing to the present invention, a wide band gap semiconductor material is formed on a substrate and doped with an effective amount of a rare earth element

The substrate itself can be any commonly used substrate such as silicon, silica, sapphire, metals, ceramics and insulators

The WBGS is either a group lll-V material or a group IV material such as diamond In particular the WBGS material can include lll-V semiconductors such as GaN, InN, AIN, BN as well as alloys of these.

Any production method which forms crystalline semiconductors can be used. Suitable tecnniques incluαe molecular beam epitaxy (MBE), metal- organic cnemical vapor deposition (MOCVD), chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PCVD), hydride vapor phase epitaxy (HVPE) and PECD. The desired thickness of a WBGS mateπal will be formed on the substrate. For emission purposes the thickness of the WBGS is not critical. For practical reasons the thickness of the WBGS layer will be from about .2 to about 5 microns, with around 1 to 2 microns being preferred.

For the rare eartn or transition metal to be strongly optically active in the wide band gap semiconductor, group III deficient growth conditioners should be utilized. This should permit the rare earth element to sit in an optically active site which promotes the higher energy or visible lig t emission

The dopant mateπal is one which has a partially filled inner shell with transition levels that can result in visible or U V emission The dopant material can be a transition metal such as chromium or a rare earth element preferably from the lanthanide series and can be any of

cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, turbium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetium Typically RE dopants include thulium for a blue display, praseodymium for a red display, and erbium for a green display These can be added to the WBGS by either in situ methods or by ion

implantation Generally the concentration can be relatively high, from less than about 0 1 % up to about 10 atomic percent The dopant concentration can be increased until the emission stops. Generally, the preferred concentration will be about .1 to about 10 atomic percent.

Further a full-color display can be created by utilizing three overlapping WBGS layers such as GaN each layer doped with different light emitting rare earth elements. Separate wiring could be used for each layer and each layer could be separated by transparent insulating layers.

An array of side by side light emitting diodes could also be used to provide a full color display. A combination of dopants in the same WBGS can also be employed. It may be desirable to anneal the WBGS. This tends to increase emission up to a point. Generally the WBGS is annealed in an argon or other inert environment at a temperature 800-1200° C for 1-5 hours. More preferably the temperature will be from 850-1050° C, most preferably about 950° C. The invention will be further appreciated in light of the following

detailed example. Example 1.

An erbium-doped GaN Schottky contact LED emitting visible light was formed by growing an erbium-doped GaN film in a Riber MBE-32 system on a two inch pSi substrate. Solid sources are employed to supply the gallium (7 N purity) and erbium (3 N) fluxes while an SVTA rf plasma source is used to generate atomic nitrogen. In this application, a GaN buffer iayer was first deposited for 10 minutes at a temperature of 600°C followed by GaN growth at a temperature of 750^CC. The growth conditions were as follows- nitrogen flow rate 1.5 seem at a plasma power of 400 Watts, gallium cell temperature of 922⁵C (corresponding to a beam pressure of 8.2 * 10^'7 torr) and erbium cell temperature of 1100°C. The resulting GaN growth rate was about 0 8 microns/hour, and the erbium concentration was about 10²⁰/cm³ GaN films with a thickness of 2.5 microns were utilized.

To fabricate Schottky diodes on the GaN-erbium films, a semitransparent aluminum layer was deposited by sputtering. The aluminum film was patterned into a series of ring structures of varying areas utilizing a lift-off process. The aluminum rings serve as individual

Schottky contacts while a large continuous aluminum surface was used as a common ground electrode. Electro luminescence characterization at ultraviolet and visible wavelengths was performed with a 0.3 m Acton research spectrometer fitted with a photo multiplier tube detector. All measurements were conducted at room temperature using dc applied bias voltage and current.

Applying reverse bias current to the order of 1 milliamp to a GaN. erbium Schottky LED, results in green emission visible with the naked eye under normal ambient lighting conditions. The emission spectrum consists of two strong and narrow lines at 537 and 558 nm which provides the green emission color The two green lines have been identified as erbium transmissions from a ²H._1/2 and ^ΛS^ levels to the ^Λ\,_5/2 ground state. Photo luminescence characterization of the same GaN erbium films grown on silicon performed with a heiium cadmium laser excitation source at a wavelength of 325 nanometers, corresponding to an energy greater than a GaN band gap, also produce green emissions from the same two transitions. Minor EL peaks were observed at 413 and at 666/672 nanometers. The device had a threshold voltage for forward conduction of about

8.5 volts. At a forward voltage of 20 volts, a current flow of 350 milliamps is obtained. Under reverse bias of 20 volts, a current of about 30 microamps is measured. The capacitance voltage characteristic of the diode has a voltage intercept of about 11.5 volts and an effective GaN carrier concentration of approximately 10¹²/cm³. The high diode forward resistence obtained in the current voltage characteristics of about 34 kilo- Ohms is probably due to the high resistivity of the GaN layer. The Schottky barrier height calculated from the capacitance voltage characteristics is approximately 9 volts, which was consistent with the threshold voltage. This large voltage probably indicates the presence of an insulating layer on the aiuminum-GaN interface.

A linear relationship is maintained between the optical output and

the bias current over a wide range of values. At current values smaller than 200 milliamps, the relationship is linear. Example 2.

Er-doped GaN films are formed in a Riber MBE-32 system on c- axis sapphire substrates. Solid sources were employed to supply the Ga (7 N purity), Al (6 N), and Er (3 N) fluxes, while an SVTA Corp. rf plasma source was used to generate atomic nitrogen. The substrate was initially nitπded at 750 ⁼ C for 30 mm at 400 W rf power with a N₂ flow rate of 1.5 seem, corresponding to a chamber pressure of mιd-10^'° Torr. An AIN buffer layer was grown at 550 °C for 10 minutes with an Al beam pressure of 2.3 * 10-⁸ Torr (cell temperature of 970° C). Growth of the Er-doped GaN proceeded at 750 ^GC for 3 hours with a constant Ga beam pressure of 8.2 * 10^"7 Torr (cell temperature of 922 °C). The Er cell temperature was varied from 950 to 1100^CC The resulting GaN film thickness was nominally 2

giving a growth rate of 0.8 um/h, as measured by scanning electron microscopy (SEM) and transmission optical spectroscopy Photoluminescence (PL) characterization was performed with two excitation sources: (a) above the GaN band gap-HeCd laser at 325 nm (4-8 mW on the sample); (b) below the GaN band gap-Ar laser at 488 nm (25-30 mW). The PL signal was analyzed by a 0.3 m Acton Research spectrometer outfitted with a photomultiplier for ultraviolet (UV)- visible wavelengths (350-600 nm) and an InGaAs detector for infrared

(1 5 urn) measurements The PL signal of the Er-doped GaN samples was obtained over the 88-400 K temperature range. Above band gap excitation (He-Cd laser) resulted in light green emission form the Er- doped GaN films, visible with the naked eye. Two major emission multiplets are observed in the green wavelength region with the strongest lines at 537 and 558 nm A broad emission region is also present, peaking in the light blue at 480 nm. The

yellow band typically observed at -540-550 nm in GaN PL is absent. Example 3.

Pr-doped GaN films were grown in a Riber MBE-32 system on 2" inch (50 mm) p-Si (111 ) substrates. Solid sources were employed to supply the Ga and Pr fluxes, while an SVTA rf-plasma source was used to generate atomic nitrogen. The growth of GaN:Pr followed the procedure previously discerned for GaN:Er. Substrate growth temperature was kept constant at 750° C and the Pr cell temperature was

1200° C. We estimate, based on our work with GaN:Er, that this cell temperature results in a Pr concentration in the range of 10¹⁸-10²⁰/cm³. PL characterization was performed with He-Cd and Ar laser excitation sources at wavelengths of 325 and 488 nm, respectively. The PL and EL signals were characterized with a 0.3-m Acton Research spectrometer outfitted with a photomultiplier tube (PMT) detector for UV-visible wavelengths and an InGaAs detector for IR. To measure EL characteristics, contacts were formed by sputtering a transparent and

conducting indium-tin-oxide (ITO) layer onto the GaN:Pr structure.

He-Cd PL excitation (as 325 nm) resulted in an intense, deep red emission from the Pr-doped GaN, visible with the naked eye. The room temperature PL at visible wavelengths is shown in Fig. 1 for a 1.5 μm thick GaN film grown on Si. The spectrum indicates a very strong

emission line in the red region at 650 nm, with a weak secondary peak at

668 nm. Example 4.

Praseodymium implantation was performed in a MicroBeam 150 FIB system utilizing a Pr-Pt liquid alloy ion source (LAIS). The Pr-Pt alloy was prepared by mixing praseodymium and platinum at an atomic percent ratio of 87.13. This produces an eutectic alloy with a melting point of 718°

C. Mass spectrum analysis showed that a Pr^2* target current of -200 pA was produced, representing 75% of the total target current. A Pt⁺ target current of -50 pA was also observed.

The Pr^2* beam was accelerated to high voltage and implantation was carried out at room temperature on GaN films grown by MBE, HVPE, and metalorganic chemical vapor deposition (MOCVD). After FIB implantation, the samples were annealed under different conditions. PL measurements were performed at room temperature by pumping the samples with a CW He-Cd laser at 325 nm. The He-Cd laser was focused on the sample surface, where the laser power and beam diameter were 12mW and 200μm, respectively. The PL signal was collected by a lock-in amplifier and characterized with a 0.3-m Acton

Research spectrometer outfitted with a photomultipiier tube (PMT) detector for UV-visible wavelengths and an InGaAs detector cooled to 0° C for IR. A grating of 1200 grooves/mm with a resolution of 1.67nm/mm was used for UV-visible wavelengths.

The Figure shows the annealing effect on PL intensity for a Pr- implanted GaN film grown on sapphire by MBE. The implanted pattern is a 136μmx136μm square. The implantation was performed using a 300keV Pr^2* beam with a target current of 200pA. The pixel exposure time was 1.14ms and the pixel size was 0.265μmx0.265μm. This results in a dose of -1x10¹⁵ atoms/cm². Simulation of these implantation conditions using TRIM'95⁹ calculates a projected range of -60 nm and a peak concentration of -1.7x10²⁰ atoms/cm³. The sample was first annealed at 950° C for one hour in flowing argon. After this first anneal, the 650nm peak became discernible. The sample was subsequently annealed at 950° C for another two hours, leading to an increase in the peak intensity at 650nm. The third anneal was carried out at 1050° C for one hour resulting in the PL intensity at 650nm increasing by a factor of

4. In spite of the small implanted pattern size (136μmx136μm), the emitted red light intensity was strong enough to be easily seen with the naked eye. Annealing for a fourth and final time at 1050° C resulted in a reduced PL intensity. This suggests that a one-step annealing at 1050° C is adequate to optically activate the Pr^3* ions implanted in the GaN film.

Similar PL spectra were observed from Pr-doped sulfide glasses. Example 5.

A GaN region was also patterned by Pr FIB implantation. The implantation was performed using a 290keV Pr^2* beam for a dose of -4.7x10¹⁴ atoms/cm². After FIB implantation, the sample was annealed at 1050°C for one hour in Ar. Under UV excitation from the He-Cd laser, the implanted region emits red light, while unimplanted surrounding area shows the yellow band emission of GaN. Exampie 6

Pr implantation was also performed on GaN films grown by HVPE and MOCVD Regions consisting of 141 μmx141 μm squares were implanted on both samples with a dose of 1x10¹⁵ atoms/cm² and a beam energy of 290keV Both samples as well as a Pr-implanted MBE sample

(dose = 4 7x10¹⁴ atoms/cm²) show strong red emission at 650nm, which corresponds to the ³P₀— ³F₂ transition of Pr^3* All three samples show similar band edge emission at around 365nm

Example 7 Pr-implanted GaN film grown by MBE on sapphire was formed.

After FIB implantation with a dose of 4 7x10¹⁴ Pr/cm² the sample was annealed at 1050°C for one hour in Ar The Pr concentration of the in-situ doped GaN film is estimated to be at the range of 10¹⁸ - 10²⁰ atoms/cm³.

In general, the PL intensity of the in-situ Pr-doped GaN sample is stronger (~5x) than that in the FIB-implanted sample, which is expected from the much larger Pr-doped volume which is excited in the former case For the samples, the full width at half maximum (FWHM) of the 648 and 650nm lines are -1 2nm, which corresponαs to 3.6meV

Thus the present invention can be utilized to produce light emitting devices from wide band gap semiconductor material utilizing rare earth dopants The particular wavelength of emission is certainly characteristic of the added component Further, it is possible to combine the rare earth implants to develop unique light emitting devices Thus the present invention lends itself to a wide variety of different light emitting devices, extending from the infrared range down through the ultraviolet range.

This has been a description of the present invention along with a preferred method of practicing the invention. However, the invention itself should be defined only by the appended claim, wherein we claim:

Claims

1. A light emitting device which emits light in the visible to UV spectrum comprising a semiconductor material selected from one of groαp lll-V and group IV wide band gap semiconductors and having embedded in there a light emitting element in an amount effective to provide emission in the visible to UV spectrum, said element comprising a light emitting element having a partially filled inner transition level suitable for light emission in the visible spectrum or higher.

2. The device claimed in claim 1 wherein said wide band gap semiconducting material selected from the group consisting of gaiium nitride, aluminum nitride, indium nitride, boron nitride, alloys thereof and diamond.

3. The light emitting device claimed in claim 2 wherein said light emitting element is selected from the group consisting of transition metals and rare earth elements.

4. The device claimed in claim 3 wherein said light emitting element is selected from the group consisting of rare earth elements.

5.- The light emitting device claimed in claim 1 wherein said semiconductor includes from about 0.1 to about 10 atomic percent rare earth element.

6. The light emitting device claimed in claim 4 wherein said rare earth element is selected from the group consisting of cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

7. The light emitting device claimed in claim 6 wherein said rare earth element is selected from the group consisting of thulium, praseodymium, and erbium.

8. The light emitting device claimed in claim 1 wherein said semiconductor material is GaN.

9. The light emitting device claimed in claim 5 wherein said semiconductor material is formed by molecular beam epitaxy.

10. The light emitting device claimed in claim 4 having at least two separate rare earth elements.

11. The light emitting device claimed in claim 1 wherein said light emitting element is chromium.

12. A method of forming a light emitting device which will emit in the visible to UV spectrum at room temperature comprising forming a semiconductor material wherein said material is selected from a group consisting of group lll-V and IV wide band gap semiconductors and adding to said semiconductor a light emitting element in an amount effective to provide visible light emission wherein said light emitting element has a partially filled transition level effective to emit light in the visible to UV spectrum.

13. The method claimed in claim 12 wherein said light emitting element is combined in situ with as said semiconductor is formed.

14. The method claimed in claim 12 wherein said light emitting element is implanted in said semiconductor material.

15. The method claimed in claim 12 wherein said semiconductor material is annealed at a temperature and time effective to enhance the visible emission of said device.

16. The method claimed in claim 12 wherein said light emitting element is chromium.

17. The method claimed in claim 14 wherein said semiconductor material is annealed after said light emitting element is combined with said semiconductor mateπal.

18. The method claimed in claim 12 wherein said light emitting element is selected from the group consisting of cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium and mixtures thereof.

19. The method claimed in claim 18 wherein said light emitting element is selected from the group consisting of thuiium, praseodymium, and erbium.

20. The method claimed in claim 12 wherein said semiconductor material is GaN.

21. The method claimed in claim 18 wherein at least two different rare earth elements are combined with said semiconductor material.

22. The method claimed in claim 18 wherein three separate rare earth elements are combined with said semiconductor material.

23. A light emitting device comprising these overlapping semiconductor layers selected from the group consisting of AIN, BN, GaN, InN and alloys thereof wherein each of said layers includes an amount of a separate rare earth element effective to provide light emission of each of said layers at different wavelengths.