The light ion impurities C, O and H have been implanted or diffused into GaN and related compounds and their effect on the electrical properties of these materials measured by Hall, C-V and SIMS as a function of annealing temperatures from 300-1100ºC. While C in as-grown GaN appears to create an acceptor under MOMBE conditions, implanted C shows no measurable activity. Similarly, implanted O does not show any shallow donor activity after annealing at ≤ 700°C, but can create high resistivity regions (106 Ω/□) in GaN, AlInN and InGaN for device isolation when annealed at 500–700°C. Finally, hydrogen is found to passivate shallow donor and acceptor states in GaN, InN, InAIN and InGaN, with dissociation of the neutral complexes at >450°C. The liberated hydrogen does not leave the nitride films until much higher annealing temperatures (>800°C). Typical reactivation energies are ∼2.0eV for impurity-hydrogen complexes.

Both infrared absorption (IR) and electron spin resonance (ESR) spectroscopies have been used to investigate the complicated structure of nanocrystalline cubic boron nitride/amorphous hydro-genated boron nitride thin films. The ESR spectra from this material consist of a component with a four-line hyperfine structure and/or a component with a ten-line hyperfine structure superimposed upon a broad central line. The hyperfine structures are associated with defect centers located in the nanocrystalline phase, whereas the broad line is attributed to dangling bonds in the amorphous phase. The IR spectra consist of three lines around 1400 cm−1: the lines at 1263 and 1505 cm−1 originate in a boron-poor amorphous hydrogenated boron nitride region; the line at 1371 cm−1, in a boron-rich amorphous hydrogenated boron nitride region. These results, together with previously reported electron diffraction spectra, suggest the following picture: small (2.5 nm) nanocrys-tallites of cubic boron nitride (about 5% of the material) are imbedded in a mixed amorphous phase. The amorphous region can be approximated by a mixture of boron-rich and boron-poor amorphous hydrogenated boron nitride.

Magneto-studies have been carried out for several MOCVD grown GaN thin films and GaN/AlGaN heterostructures at magnetic fields up to 30 T and at temperatures between 4.2 K to 100 K. Electron cyclotron resonance was observed in two heterostructures with high mobilities (μ > 2000 cm2/V-s), the effective mass obtained from the cyclotron resonance measurement is 0.23±0.01 m0, where m0 is the mass of free electron. For Si-doped thin film GaN there was no sign of electron cyclotron resonance even when samples were heated up to 100 K. However, a Is to 2p+ absorption line was observed for Si-doped GaN samples. A binding energy of 29 meV and low frequency dielectric constant of 10.4 is obtained.

Deep levels in insulating GaN grown by metalorganic chemical vapor deposition have been studied using thermally stimulated current (TSC) and photocurrent (PC) spectroscopies. Five main traps were observed by TSC measurement in the as-grown undoped GaN in the range of 0-0.75 eV below the conduction band edge or above the valence band edge. Their activation energies were 0.11, 0.24, 0.36, 0.53 and 0.62 eV, respectively. PC measurements showed three deep levels located within the bandgap at 1.32, 1.70 and 2.36 eV, respectively. Furnace annealing was carried out on GaN for identifying all the observed deep levels. We have found that the 0.24, 0.36 and 0.53 eV traps were eliminated by annealing at 1000°C under N2for six hours, whereas the 0.62 eV trap density increased after annealing. The three deep levels detected by the PC measurement were not affected by annealing. The 1.70 eV trap, which is located at the midgap, does not seem to compensate with narrow donors. We attribute the 0.11 eV trap to surface states, and the 0.62 eV trap to nitrogen vacancies.

We have performed time resolved photoluminescence measurements of the exciton recombination in different GaN samples at low temperatures. In epitaxial layers the decay time of the free exciton is typically faster than 100 ps. This is due to a dominating non-radiative recombination process. In thick bulk samples we have resolved and measured the decay time of the free exciton with a value of about 200 ps. We believe that this value is close to the radiative lifetime for free excitons in GaN. We have also shown that excitation transfer occurs between free and bound exciton states. We have furthermore measured the decay of the donor and acceptor bound excitons, and obtained values of the decay time of 250 ps and 1200 ps, respectively.

The chemical inertness and high bond strengths of the III-V nitrides lead to slower plasma etching rates than for more conventional III-V semiconductors under the same conditions. High ion density conditions (>3×1011cm−3) such as those obtained in ECR or magnetron reactors produce etch rates up to an order of magnitude higher than for RIE, where the ion densities are in the 109 cm−3 range. We have developed smooth anisotropic dry etches for GaN, InN, AlN and their alloys based on Cl2/CH4/H2/Ar, BCl3/Ar, Cl2/H2, C12/SF6, HBr/H2 and HI/H2 plasma chemistries achieving etch rates up to ∼4,000Å/min at moderate dc bias voltages (≤-150V). Ion-induced damage in the nitrides appears to be less apparent than in other III-V’s. One of the key remaining issues is the achievement of high selectivities for removal of one layer from another.

Hydrogen is implicated in the low doping efficiency of acceptors (e.g., Mg) in MOCVD-grown GaN by the following observations: the well-established role of hydrogen in the same phenomenon in other semiconductors; re-passivation of thermally activated Mg in GaN upon exposure to NH3 or H at temperatures > 400°C; correlation between Mg and H concentrations; and computational studies on the diffusivities of charged H in GaN and on the configuration and stability of the Mg-H complex in GaN. Strong experimental evidence for the Mg-H complex comes from variable-temperature Hall effect measurements which reveal that after hydrogenation of activated GaN:Mg the hole mobility increases as the hole concentration decreases, which is consistent with removal of ionized scattering centers and not with the introduction of separate compensating defects. However, experimental evidence is also accumulating to suggest that the Mg activation process involves the generation or transformation of electrically active defects.

In this paper, we present ensemble Monte Carlo based calculations of electron initiated impact ionization in bulk zincblende GaN using a wavevector dependent formulation of the interband impact ionization transition rate. These are the first reported estimates, either theoretical or experimental, of the impact ionization rates in GaN. The transition rate is determined from Fermi’s golden rule for a two-body screened Coulomb interaction using a numerically determined dielectric function as well as by numerically integrating over all of the possible final states. The Monte Carlo simulator includes the full details of the first four conduction bands derived from an empirical pseudopotential calculation as well as all of the relevant phonon scattering mechanisms. It is found that the ionization rate has a relatively "soft" threshold.

Exposure to numerous acids and bases and UV/O3 oxidation were used to determine the best ex situ cleaning techniques for the (0001) surfaces of AIN and GaN. HF and HCI were the most effective in removing the oxide from AIN and GaN, respectively. However, AES and XPS revealed the surfaces to be terminated with F and CI which inhibited re-oxidation prior to loading into vacuum. TPD showed mat temperatures of 650 and 850°C are necessary to thermally desorb the CI and F, respectively. UV/O3 oxidation in air was not effective in removing hydrocarbons from either surface but was effective for oxide growth. In situ remote hydrogen plasma exposure at 450°C removed halogens and hydrocarbons remaining after ex situ cleaning of both AIN and GaN surfaces; however, oxide free surfaces could not be achieved. Thermal desorption of hydrocarbons from GaN in UHV was achieved at 650°C. Complete thermal desorption of the surface oxide in UHV was only achieved at temperatures > 800°C where some GaN decomposition occurred. Annealing GaN in NH3 at 700°C reduced the surface oxide without loss of surface stoichiometry.

Development of devices based on the wide gap semiconductor gallium nitride (GaN) requires the realization of reliable, high fidelity, low damage pattern transfer processes. In this work, GaN thin films grown by OMVPE have been subjected to both chlorine- and methane/hydrogen-based etch chemistries in an electron cyclotron resonance microwave plasma reactive ion etching system. Both n-type and semi-insulating thin films have been utilized to examine the effect of these etch processes on the electronic properties of the materials. The methane/hydrogen-based etch system (CH4/H2/Ar) induced considerable changes in the electrical properties of both n-type and semi-insulating films, causing the former to become more insulating and the latter to become conducting. In both cases, the original electrical properties were recoverable after a short, high temperature anneal. In the chlorine-based etching system (Cl2), no changes in the electrical properties were observed and etch rates five times greater than in the methane/hydrogen-based system were achieved. Proposed mechanisms responsible for the observed behavior will be discussed. These results show that pattern transfer processes based in chlorine etch chemistries are more suitable for the generation of high performance GaN devices.

Electron cyclotron resonance (ECR) etching of GaN in Cl2/H2/Ar, Cl2/SF6/Ar, BCl3/H2Ar and BCl3/SF6/Ar plasmas is reported as a function of percent H2 and SF6. GaN etch rates were found to be 2 to 3 times greater in Cl2/H2/Ar discharges than in BCl3/H2/Ar discharges independent of the H2 concentration. In both discharges, the etch rates decreased as the H2 concentration increased above 10%. When SF6 was substituted for H2, the GaN etch rates in BCl3-based plasmas were greater than those for the Cl2-based discharges as the SF6 concentration increased. GaN etch rates were greater in Cl2/H2/Ar discharges as compared to Cl2/SF6/Ar discharges whereas the opposite trend was observed for BCl3,-based discharges. Variations in surface morphology and near-surface stoichiometry due to plasma chemistries were also investigated using atomic force microscopy and Auger spectroscopy, respectively.

The III-V nitrides are promising materials for use in UV-blue-green optoelectronics, high-temperature electronics, and negative-electron-affinity (NEA) electron emitter applications. In order to realize this potential, it is important to develop an etching technology for device fabrication. The stability of the III-V nitrides to harsh chemical environments makes most wet etching extremely difficult, so that dry etching alternatives are desirable. Recent experiments have shown that BCI3-based chemistries are effective for reactive ion etching of GaN and that KOH-based solutions may preferentially etch AIN from GaN. This paper reports on the use of BCI3 for etching AIN and AlGaN in addition to GaN and the creation of structures such as mesas and lines. It also examines the potential use of potassium Hydroxide (KOH) as a wet etchant of the nitrides. AIN, AlGaN, and GaN films grown by either metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) were patterned with Ni in 250 μm × 250 μm squares and 5 μm wide lines to create mesas and lines for typical light emitting diode (LED) or laser diode applications. Reactive ion etching was performed in a commercial reactor using BCI3 pressures ranging from 5 to 30 mTorr. Gas flow rates of 5 to 50 seem and RF powers of 50 to 150 W were employed. High nitride etch rates of up to 730 Å/min. were observed but lower etch rates were needed to avoid etching of the Ni mask. Smooth mesa surfaces and sidewalls were observed in scanning electron micrographs of the etched nitride structures. Mesas as small as 5 μm × 5 μm were patterned and made in this way. Lines were also made in a similar manner as narrow as 5 μm on GaN/AIN epilayers. Subsequent wet etching of these lines showed that KOH-based solutions such as AZ400K developer attack not only AIN but also GaN depending upon the quality of the film. Possibilities for using this wet etch as a defect etchant or selective etch of nitrides on SiC are discussed.

Electron cyclotron resonance (ECR) plasma etching characteristics of gallium nitride (GaN) are investigated using low pressure (4-10 mTorr) SiCl4/Ar and Cl2/H2/Ar ECR discharges. The purpose of this effort is to develop a dry etching process for making laser mirrors on GaN and to examine dry etching processes of GaN that do not require hydrogen, which is known to cause carrier compensation in GaN. The etch rate is found to increase near-linearly with increasing DC bias, and a minimum DC bias of 100V is required to initiate etching in SiCl4/Ar. We have also found that the material quality significantly affects the etch rate. The latter decreases with x-ray rocking curve half-width and increases with defect density. A reasonable etch rate of 660Ǻ/min and good surface morphologies obtained in SiCl/Ar ECR etching make this process suitable for gate recess of an FET. An etch rate of 5270Ǻ/min has been achieved in Cl2/H2/Ar plasmas. This is the highest reported etch rate of GaN so far. The smooth and vertical etch sidewalls (etch to mask selectivity of 16 is obtained) make this process promising for dry-etched laser mirrors on GaN.

Dry etch damage on n-GaN has been investigated using Pd Schottky diodes fabricated on surfaces etched by conventional reactive ion etching with SiCl4 plasma. The Schottky barrier height and ideality factor were investigated as a function of the plasma self-bias voltage. Current-voltage measurements revealed severe degradation of both the forward and reverse characteristics for plasma self-bias voltages in excess of -150 V.

The electron affinity of a semiconductor defines the relationship of the vacuum level and the semiconductor band structure. It is dependent on the atomic orbitals of the material and the surface termination. We report experimental and theoretical results that support the presence of a negative electron affinity on AlN and the Al rich AlGaN alloys. The GaN surface is found to exhibit a (positive) electron affinity of 3.3eV. The experimental measurements employ UV-photoemission spectroscopy on in situ gas-source MBE samples and on CVD samples. Theoretical results indicate that the (negative) electron affinity of AlN depends sensitively on the surface reconstruction and adatom termination. The experimental dependence of the electron affinity on alloy concentration is presented. The results indicate that AlGaN alloys with band gap similar or greater than that of diamond will exhibit a negative electron affinity. Field emission results are reported, and the characteristics are similar to those obtained from a diamond film. Issues related to cold cathode electronic devices based on NEA surfaces are noted.

We have observed a negative differential resistivity (NDR) in metal-semiconductor-metal (M-S-M) photoconductor made from unintentionally doped GaN grown by metalorganic chemical vapor deposition. The threshold field for the onset of NDR was found to be independent of the spacing of M-S-M fingers, and was measured to be 1.91×105 V/cm for GaN with an n-type carrier concentration of 1014 cm−3. We believe that the observed NDR is due to transferred electron effect in GaN. The threshold field value is very close to the value obtained from the theoretical simulation. This observation, to the best of our knowledge, is the first experimental evidence of transferred-electron effects in GaN, which is important in understanding GaN energy band structure and in the application of Gunn-effect devices using GaN materials.

We report the optical and structural properties of ion implanted GaN:Zn. Post-implant annealing up to 1100 °C was performed under flowing N2 in both a tube furnace and a rapid thermal annealing (RTA) system, with and without SiNx encapsulation layers. The implantation damage is quantified by transmission electron microscopy (TEM). Secondary ion mass spectroscopy (SIMS) detects significant rearrangement of implanted Zn only at the highest temperatures and doses investigated. Strain reduction, observed in GaN:Zn annealed at or above 975 °C by high-resolution x-ray diffractometry (HRXRD), indicates successful damage removal. The optical activation of annealed GaN:Zn is measured by photoluminescence (PL). The room temperature (RT) Zn acceptor transition at ∼430 nm is consistently observed in annealed GaN:Zn, but at low efficiency. We conclude that residual implantation damage and/or N loss during annealing limits the optical quality of implanted GaN:Zn.

The III-V nitride-containing semiconductors InN, GaN, and AIN and their ternary alloys are the focus of extensive research for application to visible light emitters and as the basis for high temperature electronics. Recent advances in ion implantation doping of GaN and studies of the effect of rapid thermal annealing up to 1100 °C are making new device structures possible. Both p- and n-type implantation doping of GaN has been achieved using Mg co-implanted with P for p-type and Si-implantation for n-type. Electrical activation was achieved by rapid thermal anneals in excess of 1000 °C. Atomic force microscopy studies of the surface of GaN after a series of anneals from 750 to 1100 °C shows that the surface morphology gets smoother following anneals in Ar or N2. The photoluminescence of the annealed samples also shows enhanced bandedge emission for both annealing ambients. For the deep level emission near 2.2 eV, the sample annealed in N2 shows slightly reduced emission while the sample annealed in Ar shows increased emission. These annealing results suggest a combination of defect interactions occur during the high temperature processing.

Ion damage build up has been measured by ion channeling in good quality epitaxial GaN films on sapphire. GaN is found to be remarkably resistant to ion damage, with extremely efficient dynamic defect annihilation occurring at liquid nitrogen temperature during ion implantation. When disorder does accumulate at doses around 1016cm−2 of 90 keV Si ions, the surface appears to be a strong sink for damage build up and possibly the nucleation of amorphous layers. Once ion disorder has been produced in GaN, it is extremely difficult to remove by annealing. GaN exhibits disordering and annealing behaviour which is somewhat similar to that in high Al-content AlGaAs.

MOCVD and MBE grown GaN were implanted with Ar, Mg, Si, Be, C, and O, and annealed in a conventional oven under flowing NH3 or N2 gas. Absorption measurements confirmed that implantation damage was annealed out after 90 minutes at a temperature of 1000 °C. Surface damage caused by NH3 annealing was evident in absorption and photoluminescence measurements for annealing temperatures of over 1000 °C. Although most of the implants showed no unique luminescence peaks, systematic changes in the relative intensities of the exciton, donor-acceptor pair, and yellow peaks were noted. The Mg implanted samples showed evidence of the acceptor bound exciton line at 3.44 eV, and a unique peak at 3.3 eV possibly due to a Mg free-to-bound transition.