We demonstrate well-controlled crystallographic etching of wurtzite GaN grown on c-plane sapphire using H 3 PO 4 , molten KOH, KOH dissolved in ethylene glycol, and NaOH dissolved in ethylene glycol between 90 and 180°C, with etch rates as high as 3.2 m/min. The crystallographic GaN etch planes are ͕0001͖, ͕1010͖, ͕101 1͖, ͕101 2͖, and ͕1013͖. The vertical ͕1010͖ planes appear perfectly smooth when viewed with a field-effect scanning electron microscope. The activation energy is 21 kcal/mol, indicative of reaction-rate limited etching.

Luminescence spectra of GaN epitaxial layers grown on sapphire display a strong intensity modulation of the below-band gap transitions and on the low-energy side of the near-band gap transition. The intensity modulation is attributed to a microcavity formed by the semiconductor-air and semiconductor-substrate interface. The microcavity effect is enhanced by using metallic reflectors which increase the cavity finesse. It is shown that microcavity effects can be used to determine the refractive index of the microcavity active material. Using this method, the GaN refractive index is determined and expressed analytically by a Sellmeir fit. © 1997 American Institute of Physics. ͓S0003-6951͑97͒00421-X͔ Microcavity effects in semiconductor optoelectronic devices have attracted much attention due to the potential of high-efficiency light-emitting diodes ͑LED͒, and low threshold lasers. 1 The enhancement of the spontaneous emission by microcavity effects has been demonstrated for resonantcavity LEDs in organic 2 as well as semiconducting 3 material systems. High-finesse GaN microcavities with distributed Bragg reflectors were recently realized by Redwing et al. 4 In the present study, the microcavity effects occurring in GaN epitaxial layers are analyzed and used for refractive index determination. Due to the refractive index step at the substrate-epilayer interface, the cavity effects are observed in GaN layers with a sufficiently small surface roughness. 5 By using metallic silver reflectors instead of the weakly reflecting semiconductor-air interface, the microcavity effects can be strongly enhanced. It is shown that the near-band gap transition of GaN is modulated on the low-energy shoulder only. In contrast, the entire band of below-band gap transitions are modulated. A new method is developed to determine the refractive index of the optically active material of microcavity structures. The usefulness of this method is demonstrated for GaN and the refractive index of GaN is expressed in analytic form by the Sellmeir equation.The GaN epitaxial layers were grown on ͑0001͒ oriented sapphire in an Emcore metal-organic vapor phase epitaxy ͑MOVPE͒ system. An initial 200-Å-thick GaN buffer layer was grown at 500°C after nitridation of the substrate. A homogenous 3-m-thick Si-doped GaN epitaxial layer (n ϭ2ϫ10 18 cm Ϫ3 ) was grown at 1050°C. After growth, the substrate was polished to allow for transmittance measurements. These measurements were performed using a broadband xenon light source. A polished sapphire substrate was used for reference measurements. The photoluminescence measurements were performed at room temperature with excitation by the 325 nm line of a HeCd laser. The very high luminescence intensity of the samples demonstrates the excellent quality and high radiative efficiency of the GaN epitaxial films. An excitation power density of 10 W/cm 2 on the sample surface was used. The luminescence was dispersed in a 0.75 nm monochromator and detected by a GaAs photo-multiplier connected to phase-sensitive amplifi...

Atomic force microscope images reveal a root-mean-square roughness ⌬dϭ16 nm for InGaN/GaN double-heterostructure laser structures with cleaved a-plane facets. The c-plane sapphire substrate cleaves cleanly along both the a and m planes. A theoretical model is developed which shows that the power reflectivity of the facets decreases with roughness by a factor of e Ϫ16 2 (n⌬d/ 0) 2 , where n is the refractive index of the semiconductor and 0 is the emission wavelength. Laser emission from the optically pumped cavities shows a TE/TM ratio of 100, an increase in differential quantum efficiency by a factor of 34 above threshold, and an emission line narrowing to 13.5 meV.

Depth profiling measurements of photoluminescence on GaN epitaxial films grown on c-plane sapphire with metalorganic chemical vapor deposition have been performed. Dry etching technique of reactive ion etching is used with reactive gas of CCl2F2/H2/Ar under an operation power of 200 W. Before and after each etching, reflectivity and photoluminescence spectra are measured. Film thickness is determined from both the scanning electron microscopy and the interference oscillations of the reflectivity spectra. An excellent steady etching rate of 19.2 nm/min is established. The photoluminescence measurements show that both the near-band-edge and the yellow luminescence remain fairly constant until the film thickness of about 700 nm, and a large drop is obtained in the ratio of near-band-edge to yellow emission intensity under about 300 nm. Analysis shows that the yellow luminescence emitters are mostly confined within the near interface region, and supports the origin of yellow luminescence to be due to native defects instead of impurities.

We demonstrate crystallographic wet chemical etching of p-type GaN with etch rates as high as 1.2 m/min. Etchants used include molten KOH, KOH dissolved in ethylene glycol, aqueous tetraethylammonium hydroxide, and phosphoric acid (H 3 PO 4), at temperatures ranging from 90 to 260ЊC. The observed crystallographic p-GaN etch planes are (0001), {101 ෆ0}, and {101 ෆ2 ෆ}. The etch rates follow an Arrhenius characteristic with activation energies varying from 21 kcal/mol for KOH-based solutions to 33 kcal/mol for H 3 PO 4. The etch rate and crystallographic nature for the various etching solutions are independent of conductivity, as shown by seamless etching of a p-GaN/undoped, high-resistivity GaN homojunction and by comparison of the etch rates of p-GaN with n-GaN.

͑Received 29 December 1997͒Near-band-gap UV and yellow band emission from metal-organic chemical vapor deposition grown GaN films on sapphires are investigated under laser excitation. The intensities of the UV and the yellow peaks increase at different rates as the entrance slit width of the spectrometer increases. The spatial distribution of the luminescence emission is analyzed through the dependence of photoluminescence intensity on the slit widths of the spectrometer. The yellow emission originates from a spot with a size about 1.5 times larger in diameter than the UV emission. Using an absorption mechanism, a Lorentzian line-shape distribution fit with the data gives estimated effective absorption coefficients of 47 cm Ϫ1 for the UV signal at 364 nm and of 32 cm Ϫ1 for the yellow signal at 546 nm, which agrees perfectly with the ones from an exponential decay fit. Dependence of UV-to-yellow peak ratio on the slit widths of the spectrometer, and consistence with possible origins of yellow luminescence is discussed. ͓S0163-1829͑98͒03132-4͔

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Optically pumped laser action is demonstrated in InGaN/GaN double heterostructure lasers with wet-etched facets. The facets are formed by a two-step etching process which creates vertical facets with less than 5 nm roughness. The first step, photoenhanced electrochemical wet etching, is used to define the laser cavities. The second step reduces the facet roughness by crystallographic wet chemical etching. Lasing is demonstrated by an increase in the differential quantum efficiency, linewidth narrowing, and strongly polarized output above threshold. The threshold varies with cavity length from 2.4 MW/cm2 for 500 μm cavities to 23 MW/cm2 for 50 μm cavities. A modal loss of 15 cm−1 is deduced from an analysis of the threshold pumping power as a function of cavity length.