JP4873705B2 - Method for forming indium gallium nitride (InGaN) epitaxial thin film having indium nitride (InN) or high indium composition - Google Patents

Description

  The present invention relates to a method for forming InN or a high In composition InGaN epitaxial thin film, and more particularly to a method for epitaxial growth of InN or a high In composition InGaN on a GaN buffer layer.

  Field-effect transistors using two-dimensional electron gas generated at light emitting diodes (LEDs), laser diodes, and heterointerfaces of GaN and AlGAN, as the epitaxial growth technology for nitride semiconductor materials with wide band gaps such as GaN and AlGaN has advanced greatly Research and development such as (HEMT, HFET) has become active, and some commercialization has begun. In particular, in light-emitting devices based on GaN and AlGaN, the role of mixed crystal materials such as GaInN, AlInN, and AlGaInN doped with indium (In) at an atomic concentration of about 10% to 20% becomes extremely important. Yes. In addition to changing the band gap by adding In, the desired emission wavelength can be obtained, the average lattice constant of the crystal can be controlled, and the lattice mismatch rate with the substrate crystal as the base of epitaxial growth can be reduced. Because there is an advantage of. In addition, since the added In atoms are naturally distributed non-uniformly in the crystal, the band gap of the portion with a large In concentration becomes small when viewed from a microscopic viewpoint, and free electrons and holes are localized there. Then, it recombines and contributes to light emission. This increases the probability that the light-emitting region is spatially isolated from dislocations and defects that are present in a large amount in nitride-based crystals, resulting in high luminous efficiency worthy of practical use despite the high dislocation and defect density. It is done.

  As described above, conventionally, a nitride crystal material having a relatively low In concentration has been studied, but recently, attention has been focused on InN and InGaN having a high In concentration. Although historical details are omitted, the band gap of InN has hitherto been considered to be in the visible light emission wavelength region of 2.1 eV or 1.9 eV. However, recently, with the progress of crystal growth technology, relatively good quality InN crystals have been obtained, and many important reports have been reported that the band gap is around 0.7 eV in the near infrared region. . Detailed investigations are still in progress to determine a highly accurate band gap value. The fact that the band gap of InN is in the vicinity of 0.7 eV is a combination of the fact that GaN and AlGaN have a band gap corresponding to the ultraviolet region. It can be covered with just that, and its technical meaning is great. In other words, application to an optical communication wavelength band with a wavelength of 1.3 to 1.55 μm, which has been obtained only with conventional As and P-based materials such as InGaAs and InGaAsP, has also come into view.

In addition, since the effective electron mass of InN is very small, high frequency operation can be expected when considering application to an electronic device, which is very attractive. Due to such a background, research and development for InN or InGaN having a large In composition has recently been greatly promoted.
However, the epitaxial thin film growth technology of InN or InGaN with a high In composition is not yet matured, and the potential (excellent physical property value) inherent in InN has not been fully utilized. Since InN has a weak atomic bonding force and a dissociation temperature under atmospheric pressure of about 600 ° C., it grows at a low temperature of 600 ° C. or lower in order to suppress the formation of defects related to N vacancy in the crystal. It is desirable. At present, an InN crystal having relatively high purity and crystallinity is obtained by using a growth technique called RF-MBE (radio frequency-molecular beam epitaxy). In RF-MBE, generates _{N 2} molecules in supplying the _{N 2} gas to the atomic N radicals and excited states to flop plasma chamber provided with the 13.56MHz high-frequency coil, which is irradiated onto the substrate And used as a raw material for elemental nitrogen. On the other hand, a general Knudsen cell (K cell) is used to supply the indium element. By the way, when epitaxial growth of InN is performed, there is no substrate crystal that can take lattice matching. For this reason, there are methods such as using sapphire (Al ₂ O ₃ ) as a substrate and growing InN directly on the substrate, or growing a GaN buffer layer on sapphire, and then growing InN. . In general, it is said that growing InN after growing a GaN buffer layer has higher crystallinity because formation of rotational domains in the growth plane is suppressed. However, even when a GaN buffer layer is used, since the flatness and crystallinity of the GaN buffer layer greatly affect the crystallinity of InN that grows thereafter, it is necessary to optimize and precisely control the growth conditions of the GaN buffer layer. there were.

The present invention provides a useful and simple growth method for obtaining an InN epitaxial thin film having excellent surface flatness and crystal structure integrity and excellent optical characteristics.
As described above, in growing an InN epitaxial thin film on a GaN buffer layer, an InN thin film having flatness on the surface of the growth layer of InN with irregularities of the order of several atomic layers and excellent in crystallinity and optical characteristics is obtained. In this case, the flatness and crystallinity of the GaN buffer layer are important. However, the crystallinity of GaN is difficult to control because it depends on the growth temperature, the V / III ratio, which is the supply ratio of the group V element (N) and the group III element (Ga), and various other growth conditions. For this reason, it has been desired to develop a technique that is as simple and effective as possible and has excellent reproducibility and controllability in performing good-quality InN epitaxial growth on the GaN buffer layer.

The important point of the present invention is that the thickness of one atomic layer to two atomic layers (ML) is obtained without supplying a plasma source of nitrogen on the surface of the GaN buffer layer prior to epitaxial growth of the InN thin film on the GaN buffer layer. It is to supply In metal. As a result, the surface morphology of the InN epitaxial growth layer becomes flat on the atomic layer order, and in addition, an increase in diffraction intensity and a decrease in half-value width investigated by X-ray are realized. In addition, the photoluminescence (PL) intensity is increased and crystallinity is improved.
In particular,

The epitaxial growth method of the invention 1 employs a configuration characterized by depositing a 1-2 ML In metal layer on the GaN buffer layer prior to the growth of the InN or high In composition InGaN layer.

In the invention 1, a configuration is adopted in which the polarity of the GaN buffer layer is N polarity ((000-1) plane, -c plane).

Furthermore, in the invention 1, as a growth technique, nitrogen atom is supplied as a means for supplying N ₂ gas to a plasma chamber provided with a high frequency coil, and plasma generated from the N ₂ gas is used, and Ga and In are supplied as means for supplying K A configuration characterized by using an atomic beam generated by heating In or Ga metal element at a high temperature in the cell was adopted. (Method called RF-MBE).

As described above, according to the present invention, when a metal In layer equivalent to 1-2 ML is deposited on a GaN buffer layer and then an epitaxial thin film of InN is grown, the surface flatness of the InN growth layer is improved to unevenness of several atomic layers. Is done.
In the future, when a nitride material having a large band gap such as GaN, AlGaN, or InGaAlN is hetero-growth on the InN layer, the flatness of the interface greatly affects the performance of the optical device and the electronic device. It is extremely in demand to produce a flat InN surface at the level. In addition, since the crystallinity of InN is greatly improved by the present invention, it greatly contributes to the improvement of the performance of optical devices and electronic devices using InN in the active layer. In the description, an InN crystal is taken as an example, but the same effect can be expected even with InGaN having a large In composition. Further, it has been described as an example RF-MBE as the growth method of InN, this method is also effective MOCVD (MOVPE) or HVPE such other epitaxial growth how InN and InGaN growth high In composition with Is easy to imagine.

An application example of the present invention will be described using RF-MBE as an example. FIG. 1 shows a diagram of RF-MBE used in the practice of the present invention. FIG. 2 shows a temperature program used for the growth of RF-MBE. The c-plane sapphire substrate was set in the MBE chamber, and cleaning was performed by heating in a vacuum at 880 ° C. for 1 hour. Subsequently, the sapphire substrate was nitrided by irradiation with a nitrogen RF plasma source for 30 minutes. The input power of nitrogen to the RF gun was 500 W, and the flow rate of N2 was 0.5 sccm. At this time, the pressure in the MBE chamber was 2.5 × 10 ⁻⁶ Torr. Thereafter, GaN was grown to about 300 nm under the same condition of 880 ° C.

  It has been confirmed by etching with KOH that this GaN has grown on the N stabilization plane (-c plane). Subsequent to the growth of GaN, the Ga cell of the K cell and the RF radical beam of N were stopped, and the substrate temperature was lowered to 500 ° C. in a vacuum. In order to examine the effect of the present invention, three types of the In metal layer deposited on the GaN buffer layer before the growth of 100 nm InN at 500 ° C., 1.8 ML, 0.9 ML deposited, and none deposited at all. Produced. The growth of InN was performed at 500 ° C., and the atomic supply ratio of In and N was slightly N-rich to prevent the dissociation of N element from the InN crystal. Thereafter, the surface morphology of InN was examined with an atomic force microscope (AFM), and a PL spectrum was measured at a sample temperature of 77K in order to examine the crystallinity of InN. An Ar ion laser of 514.5 nm was used as a PL excitation laser, and a Ge pin diode was used for detection of PL emission.

  FIG. 3 shows AFM images of three samples. In the sample of (a) in which an In metal layer of 1.8 ML is inserted at the interface between the InN layer and the GaN buffer layer, a step of about 0.55 nm is observed on the surface, and the flatness is extremely high. 0.55 nm substantially coincides with the lattice constant in the c-axis direction of the InN crystal. Half of the c-axis lattice constant (c / 2) = 0.285 nm is sometimes referred to as a bilayer layer that is a pair of In and N atomic layers, and a surface step of 0.55 nm corresponds to two bilayers. Thus, it can be seen that two-dimensional growth occurs by inserting an approximately 2 ML In metal layer. Similarly, (b) was obtained by inserting a 0.9 ML In metal layer. In this case, steps of 1.10 nm, 1.65 nm, and 2.2 nm were observed. These 1.10 nm, 1.65 nm, and 2.2 nm are considered to correspond to 4 bilayer, 6 bilayer, and 8 bilayer, respectively. Thus, the steps observed on the surface are all regular steps in units of atomic layer steps, and the movement of atoms on the growth surface is activated by the insertion of the In metal layer, resulting in a two-dimensional flat surface. It can be said that proper growth is promoted. On the other hand, in the sample (c) in which the In layer is not inserted at all, large holes as deep as 50 nm are observed on the surface, and it is considered that a three-dimensional crystal growth mode occurs instead of two-dimensional growth.

As described above, it was confirmed that when InN is grown after depositing a very thin In metal layer of 1-2 ML on the GaN buffer layer, the surface morphology is flattened at the atomic level. In addition, although it is the range of the deposition amount of the In metal layer effective in the present invention, when about 3 ML of In is inserted, the growth rate is lowered, and a very good InN crystal layer is not obtained. For this reason, it can be said that the deposition amount of the metal In layer is most effective in the vicinity of 1 ML to 2 ML.
FIG. 4 is a PL spectrum obtained from the sample described in FIG. All of the produced InN epitaxial thin films have an emission wavelength in the vicinity of 0.75 eV, indicating that the InN crystal layer has a relatively high purity. What is important is that the light emission intensity is dramatically improved by inserting an In metal layer, compared to the case where the In metal layer is not inserted. This indicates that the insertion of the In metal layer greatly improves the crystallinity of the InN epitaxial growth layer. When comparing the amounts of In deposited between 0.9 and 1.8 ML, the 1.8 ML sample has higher emission intensity, which is in good agreement with the improvement in surface flatness shown in FIG. .

Schematic diagram of an apparatus for carrying out RF-MBE of the embodiment Temperature program used for growth of RF-MBE in Example Photograph of AFM image showing working sample and graph showing surface irregularities Graph showing the PL spectrum obtained from the working sample

Claims (1)

In the epitaxial growth of InN or high In composition InGaN on the GaN buffer layer,
As supply means nitrogen atom as the growth technique, by supplying N ₂ gas to a plasma chamber in which a high-frequency coil, using a plasma generated from a N ₂ gas, In in the K cell as a means for supplying Ga and In or Ga Utilizing atomic beams generated by heating metal elements at high temperatures,
The polarity of the GaN buffer layer is N polarity ((000-1) plane, -c plane),
An epitaxial growth method characterized by depositing a 1-2 atomic layer In metal layer on a GaN buffer layer prior to the growth of an InN or high In composition InGaN layer.