The great scientific and commercial success of the group-III nitrides in recent years is the result of persistent fundamental research over a time span of three decades. In the late 60's and in the early 70's the very heart of gallium nitride research was located in J.I. Pankove's laboratory at RCA. There the first single crystalline GaN was grown by Maruska and Tietjen and the very first GaN light emitting diodes were produced by Pankove in September 1971, 26 years ago. Since then the community of nitride research has come a long and troublesome way, but it has succeeded. This 1997 Fall Meeting Symposium on Nitride Semiconductors of the Materials Research Society is dedicated to Professor J.I. Pankove for his outstanding and groundbreaking contributions in the early development of group-III nitride research. This paper reports a historical summary of the evolution of the field summarizing the landmark contributions that have led to the current status of success.

The recent progress in high pressure crystallization of GaN is reported. The results of the growth from the solutions of atomic nitrogen in pure Ga and in its alloys with Mg, Ca and Zn are discussed. It is shown that the growth mechanisms and the physical properties of the crystals depend on the type of dopant added into the solution. In particular, high resistivity (104-106Ωcm) GaN crystals of improved structural quality can be grown from solutions containing Mg. It was also observed that the addition of Mg, Ca and Zn suppress yellow photoluminescence commonly observed in GaN crystals grown without an intentional doping. The preparation of surfaces of GaN substrates for homoepitaxy by mechanical and mechanochemical polishing is discussed. It is shown that atomically flat, thermally stable surfaces are possible to obtain by the applied procedures.

Some most interesting results concerning homoepitaxial growth by MOCVD and MBE is shortly reviewed. In particular, it is shown that perfectly matched (strain free) GaN layers can be deposited on the highly resistive GaN:Mg substrates.

Thick epitaxial layers of GaN on SiC and sapphire are grown by using the sublimation sandwich method. It is shown that growth of good quality GaN crystals with the growth rates up to 0.5 mm/hour is possible using this technique. The grown layers have been separated from the seed and free standing GaN crystals up to 15 mm size were obtained.

Recent studies have demonstrated that dimethylethylamine alane (DMEAA) is a viable group III precursor for depositing high quality aluminum nitride thin films during atomic layer growth with ammonia as the group V source. However, a practical consideration that is questioned but seldom investigated is whether one should initiate the growth with the group III or the group V source. Clearly DMEAA interacts differently with silicon than does ammonia; hence, reversing the deposition sequence will lead to different interfacial composition. Earlier studies involving TMAA and ammonia indicate that direct interaction of group III precursor with the surface may lead to higher carbon contamination. In this work, adsorption of DMEAA on Si(100) and on ammonia-covered Si(100) are characterized with Temperature-Programmed Desorption (TPD), Secondary-Ion Mass Spectrometry (SIMS), and Temperature-Programmed Secondary-Ion Mass Spectrometry (TPSIMS). Preliminary results indicate that DMEAA adsorbs molecularly on both Si(100) and ammonia-covered Si(100), but to a much smaller coverage on the latter surface. Results from reversing the adsorption sequence, i.e. ammonia first then DMEAA, will be compared as a possibility for interfacial quality control.

The OMCVD of AlN, GaN and InN thin films using the novel single source precursors (N3)2Ga[(CH2)3NMe2] (1), (N3)In[(CH2)3NMe2]2 (2) and (N3)Al(CH2)3NMe2]2 (3) is reported. The compounds are non-pyrophoric. Compound 2 is air stable. No additional N-sources were used for the growth of the nitrides. We achieved epitaxial (AlN, GaN) or polycrystalline (InN) growth at least 200°C below the decomposition temperature of the respective nitride. Some aspects of the reactivity of the precursors with ammonia and the resulting influence on the deposition process were investigated.

The incorporation of nitrogen into sapphire substrates during nitridation was studied by xray photoelectron spectroscopy (XPS). An increase in the intensity of nitrogen 1s peak in XPS was observed upon longer nitridation. The surface morphology of the substrates was characterized by atomic force microscopy (AFM). High resolution electron microscopy (HREM) was employed for structural analysis. The cross sectional TEM showed a thin layer of AlN buried between amorphous AlNxO1−x and sapphire. This is the first direct observation of AlN on sapphire. The TEM images show a deeper penetration depth of nitrogen into a longer nitridated sapphire.

In this paper, we report on a systematic study of GaN growth on the A-plane sapphire by plasma-assisted MBE. The effects of plasma nitridation of the substrate and the growth of a low temperature GaN buffer on the structure and optoelectronic properties of the films are addressed. TEM studies indicate that films grown on substrates which were not nitridated prior to growth have a significant fraction of zinc-blende domains and poor orientational relation with the substrate. On the contrary, nitridation leads to films with superior structural and optoelectronic properties. The low temperature GaN buffer, grown on nitridated substrates, was found to also have a pronounced effect on the optoelectronic properties of the GaN films, especially in those with low carrier concentrations. The correlation between TEM and photoluminescence studies suggest that the transition at 3.27 eV can be attributed to the cubic domains in the films.

We have used atomic force microscopy (AFM) to study the effect of common substrate surface treatments for the metal-organic chemical vapor deposition (MOCVD) of GaN on sapphire. It appears that contaminants play a major role in the surface chemistry and strongly influence the morphology of the treated surfaces. In order to investigate the role of these contaminants, we have introduced the concept of “controlled contamination” (CC), namely, exposure of the sapphire surfaces to controlled amounts of potential contaminants in-situ and investigation of the resulting sapphire morphology. The results showed that sapphire, considered to be a very stable oxide surface, is clearly reactive in the GaN MOCVD chemical environment at the high temperatures (HT) employed, allowing us to use CC for obtaining sapphire substrates with controlled roughness. Nevertheless, epitaxial growth using the two-step GaN MOCVD process appears to be very robust and practically insensitive to the (submicronscale) substrate morphology.

The GaN formation on GaAs(100) using NH3 has been investigated using synchrotron radiation photoemission spectroscopy and an atomic force microscope (AFM). This study showed the effect of nitridation temperature on the compositional change, the chemical states, and the surface morphological changes in the nitridated surface layer. It was observed that ammonia is decomposed to an activated nitrogen atom above 700°C forming GaN on the surface. A thermally nitridated layer was composed of metallic Ga and GaN islands elongated along the [011] direction to relax the tensile strain in the [011] direction. As the nitridation temperature increased, the composition of GaN increased in the nitridated layer due to the efficient thermal decomposition of NH3 and the subsequent incorporation of the N atom to the metallic Ga. The surface morphology of the nitridated layer was also sensitive to the nitridation temperature and a smooth surface morphology could be obtained on the surface nitridated at 700°C.

High quality cubic GaN was grown on Silicon (001) by metalorganic vapor phase epitaxy (MOVPE) using a GaAs nucleation layer grown at low temperature. The influence of various nucleation conditions on the GaN epilayers' quality was investigated. We found that the GaAs nucleation layer grown by atomic layer epitaxy (ALE) could improve the quality of GaN films by depressing the formation of mixed phase. Photoluminescence (PL) and X-ray diffraction were used to characterize the properties of GaN epilayers. High quality GaN epilayers with PL full width at half maximum (FWHM) of 130meV at room temperature and X-ray FWHM of 70 arc-min were obtained by using 10–20nm GaAs nucleation layer grown by ALE.

This study investigated the low temperature growth of GaN on a nucleation layer in a remote plasma enhanced-ultrahigh vacuum chemical vapor deposition (RPE-UHVCVD) system which is equipped with an rf plasma cell for a nitrogen source. It was found that the growth temperature and the film thickness of the nucleation layer and the nitrogen flow rate for GaN growth play important roles in the improvement of crystallinity of the GaN layer. The nitridation of sapphire was also found to enhance the formation of facet shaped nuclei on the nucleation layer. As the temperature of the nucleation layer increased, islands with hexagonal and other facet shapes were formed on the grown GaN surface. This facet formation was related with the surface morphology and crystallinity of GaN. The best crystallinity was measured in a GaN layer with hexagonal facets on the surface and such GaN layers could be grown on a nucleation layer grown at 375 °C. Nitridation of sapphire and the growth temperature of the nucleation layer were also found to change the island shapes which enhances the formation of columnar structures in the GaN layer, resulting in the growth of a high crystalline GaN layer at low temperature.

A remote plasma enhanced-ultrahigh vacuum chemical vapor deposition (RPE-UHVCVD) system equipped with a radio frequency-inductively coupled plasma (RF-ICP) which produces the reactive nitrogen species was employed to study the nitridation process at low temperature. The sapphire surface nitridated under various conditions was investigated with x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The nitridation process seems to be mostly affected by the RF power even at low temperature since the intensity of the N1s, peak was not dependent on the substrate temperature but on the RF power. The AFM images showed that the protrusion density on the sapphire surface decreased rapidly when the nitridation temperature was decreased. This result suggests that the formation of the protrusions is closely related to the process temperature, indicating that the formation of such protrusions is caused by the change of an elastic strain energy due to the thermal stress. It was possible to nitridate the sapphire surface without protrusion at a very low temperature. The crystallinity of GaN grown at 450 °C was found to be much improved when the sapphire substrate was nitridated at low temperature prior to the GaN layer growth.

The evolution of the microstructure of GaN grown by molecular beam epitaxy on {001} and {111} oriented GaAs substrates has been followed using transmission electron microscopy and reflection high energy electron diffraction. A thin layer of GaN has been shown to form during the nitridation of the GaAs surface prior to growth. Growth of GaN then proceeds by an island mechanism. Faulting on the four {111} planes of the cubic zinc-blende phase which grows on the {001 } surface occurs at an early stage as a consequence of misfit strain. The distribution of the {111} microtwins is initially isotropic, but growth of one pair of {111} twins proceeds much faster than that of the other pair, leading to a final microstructure which has an anisotropic distribution of microtwins. Doping of GaN with Si hinders the growth of the zinc-blende phase, leading to a textured, columnar (0001) wurtzite microstructure. Evidence is presented to show that addition of Mg as a dopant may reduce the stacking fault energy of wurtzite GaN.

The nucleation and initial stage of GaN growth on sapphire was investigated by atomic force microscopy, X-ray diffraction and photoluminescence. A 15 to 30 nm thick GaN buffer layer deposited at proper conditions was extremely smooth and nearly amorphous. Proper post deposition annealing resulted in the buffer crystallized. The buffer layer deposition temperature, thickness and annealing time and temperature must be coordinated. Low deposition temperature and/or insufficient annealing of the buffer results in a GaN wafer which has fine spiking surface morphology with an RMS of 3.4 nm for 1.4 μm wafer, strong yellow luminescence and wide xray rocking curve FWHM. High deposition temperature, longer crystallization time, and a low growth rate results in a wafer which exhibits strong band edge luminescence without noticeable yellow luminescence, and a narrow (002) diffraction rocking curve. However, the surface morphology exhibits well developed hexagonal feature with RMS roughness of 14.3 nm for a 570 nm thick layer. X-ray rocking curve analysis revealed buffer crystallization, domain coalescence and alignment process. The FWHM of the ω–scan of GaN (101) diffraction was 1700–2000 arc seconds for 200–1400 nm wafers which indicates that the twist of the domains is not changing much with the growth.

Using a novel ultrahigh vacuum transmission electron microscope (UHV TEM) with insitu molecular beam epitaxy capability we have studied the nitridation of (0001) sapphire upon exposure to ammonia. Atomically flat sapphire surfaces for the experiments were obtained by high temperature annealing. Subsequent exposure to ammonia flow at 950°C led to the successful synthesis of epitaxial AIN; the films were characterized in-situ using TEM. Complimentary ex-situ atomic force microscopy (AFM) was also performed in order to characterize the surface morphology before and after nitridation.

The experiments indicate that AIN grows by a 3D island growth mechanism. Electron diffraction patterns suggest an abrupt AIN/sapphire interface with no evidence of the formation of Al–O–N compounds. The rate limiting step in the nitridation reaction appears to be the diffusion of nitrogen and oxygen species between the free surface of the growing AIN film and the reaction interface. It is inferred from kinetic measurements that diffusion of these species occurs along the boundaries between coalescing AIN islands.

InGaN films and InGaN/GaN quantum wells with high indium content have been grown by MOVPE and characterised to evaluate the growth process and the indium incorporation efficiency. The characterisation techniques include photoluminescence, DC X-ray and TEM. The closed spaced vertical rotating disk reactor configuration results in a very high Indium incorporation for InGaN material, compared to other configurations. InGaN layers with an indium composition up to 56 % have been deposited which still exhibit very good optical properties (intense PL emission). The influence of various growth conditions on the InGaN composition and quality have been investigated to optimize the layer quality. TEM diffraction patterns have shown that the ternary InGaN layer can be chemically ordered. The In and Ga atoms occupy respectively the two simple hexagonal sublattice sites related by the glide mirrors and helicoidal axes of the P6 3mc symmetry group of the wurtzite GaN.

Mg-doped GaN films with a variety of Mg concentrations were grown on sapphire (0001) by horizontal atmospheric metalorganic chemical vapor deposition (MOCVD) system with three layered laminar flow gas injection in an attempt to study the Mg doping effects on film quality. The increase of Mg concentration induced an increase of x-ray rocking curve full width at half maximum (FWHM) and degradation of surface morphology. Secondary ion mass spectroscopy (SIMS) analysis shows increase of Si and O, associated with Mg-doping concentration. Si and O concentrations of Mg-doped film are up to 5×1016cm−3 and 5×1017cm−3 at Mg concentration of 4.5×1019cm−3, respectively. Strong 380nm emission and weak 430nm emission were observed by photoluminescence (PL) measurement at room temperature for as-grown Mg-doped GaN films which shows p-type conductivity after thermal annealing. While, in highliy Mg-doped GaN films which do not show the p-type conduction after thermal annealing, 430nm and/or 450nm emission were dominating. The highest room temperature free hole concentration achieved was p=2.5× 1018cm−3 with mobility μp=l.9cm2/V s.

We investigated the growth conditions for enhancing epitaxial lateral overgrowth (ELO) of GaN stripes selectively grown by low-pressure metalorganic vapor phase epitaxy. The ELO was enhanced for GaN <1100> stripes and with a small trimethylgallium flow-rate. This tendency did not depend on mask materials. The cross-sectional shape of the GaN <1100> stripes was trapezoidal for SiO2 masks, and rectangular for silicon nitride (SiN2) masks in a certain growth condition. A low dislocation density in the ELO region was obtained not only for the SiO2 masks but also for the SiN2 masks.

InGaN films in the 0–50% InN composition range have been analyzed for the occurrence of phase separation. The ñ0.5 jum thick InGaN films were grown by metalorganic chemical vapor deposition (MOCVD) in the 690 to 780°C temperature range and analyzed by θ−20 x-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area diffraction (SAD). As-grown films with up to 21% InN were single phase. However, for films with 28% InN and higher, the samples showed a spinodally decomposed microstructure as confirmed by TEM and extra spots in SAD patterns that corresponded to multiphase InGaN. An explanation of the data based on the GaN-InN pseudo-binary phase diagram is discussed.

ZnO films are being used in many emerging technologies such as high frequency RF filters, substrate/buffer layer for GaN deposition, and as active layers in blue-UV LEDs and lasers. These applications require epitaxial films with high structural quality. In the present paper, results on the structure of ZnO films deposited by MOCVD on c-plane and r-plane sapphire substrates are presented. These films have been investigated by XRD, SEM and TEM. X-ray phi-scans show that the films have a single epitaxial relationship with each substrate orientation. (11.0) ZnO grows on (01.2) sapphire (r-plane), while (00.1) oriented films grow on (00.1) sapphire (c-plane). Smooth films are obtained on r-sapphire, while films with a columnar morphology are obtained on c-sapphire. Based on the morphology of the columnar grains grown on c-sapphire, it is expected that the films are oxygen terminated. The interface between ZnO and sapphire is atomically sharp as observed by HRTEM. Misfit dislocations at the interface between ZnO and r-sapphire have been observed. Grain boundaries between coalescing islands during film growth are the main type of defects. Under certain conditions, very long whiskers were observed to grow by the VLS mechanism. By modifying our system, we have been able to prevent the growth of these whiskers.