S. D. Hersee

Nonalloyed Ti/Al Ohmic contacts to n‐type GaN using high‐temperature premetallization anneal

On Si‐implanted n‐type GaN, a nonalloyed Ti/Al metallization has been found to form an Ohmic contact that has a specific contact resistance as low as 1.0×10−5 Ω cm2. The Ohmic character is believed to be caused by the 1120 °C implant activation anneal which generates nitrogen vacancies that leave the surface heavily n type. This theory is indirectly confirmed on unimplanted n‐type GaN by comparing the rc of nonalloyed Ti/Al on unannealed GaN with that of nonalloyed Ti/Al on 1120 °C annealed GaN. The former has rectifying electrical characteristics, while the latter forms an Ohmic contact with an rc=1.3×10−3 Ω cm2.

Broadly tunable external cavity laser diodes with staggered thickness multiple quantum wells

Widely tunable low-threshold current laser diodes fabricated from an engineered multiple-quantum-well (MQW) gain structure consisting of three compressively strained In/sub 0.2/Ga/sub 0.8/As wells of different thicknesses are reported. Using a grating in an external cavity, a continuous-wave tuning range of 70 nm (911-981 nm) is measured for a 155-/spl mu/m semiconductor cavity length device at a current of 32 mA. This is the lowest reported bias current for a semiconductor laser with this broad a tuning range. A maximum continuous wave tuning of 80 nm (901-981 nm) has been measured at a bias current of 95 mA. At long wavelengths, a suppression of amplified spontaneous emission and preferential population of the lowest energy well were observed.

Morphology and photoluminescence improvements from high-temperature rapid thermal annealing of GaN

Rapid thermal annealing of GaN in an Ar or N2 ambient up to 1100 °C is shown to improve surface morphology and photoluminescence intensity. For both ambients the average rms surface roughness as determined by atomic force microscopy decreases from ∼4 nm on the as‐grown material to ∼1 nm after a 1100 °C anneal. The band‐edge luminescence intensity was increased by a factor of 4 after a 1100 °C anneal in a N2 ambient and a factor of 2 for annealing at 1100 °C in an Ar ambient as compared to as‐grown material. The 1100 °C anneal improves the ratio of band edge to deep‐level luminescence and also reduces the electron concentration and mobility. The reduction in mobility can be explained in terms of a two‐band conduction mechanism where defect band conduction dominates at the lower carrier densities or an increase in the free‐carrier compensation ratio.

Effect of threading defects on InGaN∕GaN multiple quantum well light emitting diodes

Photoelectrochemical etching was used to measure the threading defect (TD) density in InGaN multiple quantum well light-emitting diodes (LEDs) fabricated from commercial quality epitaxial wafers. The TD density was measured in the LED active region and then correlated with the previously measured characteristics of these LEDs. It was found that the reverse leakage current increased exponentially with TD density. The temperature dependence of this dislocation-related leakage current was consistent with a hopping mechanism at low reverse-bias voltage and Poole-Frenkel emission at higher reverse-bias voltage. The peak intensity and spectral width of the LED electroluminescence were found to be only weakly dependent on TD density for the measured TD range of 1×107–2×108cm−2.

Temperature Behavior of Pt/Au Ohmic Contacts to p-GaN

Ohmic contacts to Mg-doped p-GaN grown by MOCVD [1] are studied using a circular transmission line model (TLM) to avoid the need for isolation. For samples which use a p-dopant activation anneal before metallization, no appreciable difference in the specific contact resistance, r c , as a function of different capping options is observed. However, a lower r c is obtained when no pre-metallization anneal is employed, and the post-metallization anneal simultaneously activates the p-dopant and anneals the contact. This trend is shown for Pt/Au, Pt, Pd/Pt/Au, and Ni/Au contacts to p-GaN. The r c s for these metal contacts are in the range of 1.4–7.6 × 10 -3 ohm-cm 2 at room temperature at a bias of 10mA. No particular metallization formula clearly yields a consistently superior contact. Instead, the temperature of the contact has the strongest influence. Detailed studies of the electrical properties of the Pt/Au contacts reveal that the I-V linearity improves significantly with increasing temperature. At room temperature, a slightly rectified I-V characteristic curve is obtained, while at 200°C and above, the I-V curve is linear. For all the p-GaN samples, it is also found that the sheet resistance decreases by an order of magnitude with increasing temperature from 25°C to 350°C. The specific contact resistance is also found to decrease by nearly an order of magnitude for a temperature increase of the same range. A minimum r c of 4.2 × 10 -4 ohm-cm 2 was obtained at a temperature of 350°C for a Pt/Au contact. This result is the lowest reported r c for ohmic contacts to p-GaN.

Nanoheteroepitaxy: Nanofabrication route to improved epitaxial growth

Nanoheteroepitaxy is a fundamentally new epitaxial approach that utilizes three-dimensional stress relief mechanisms available to nanoscale heterostructures to eliminate defects provided the island diameter is below a critical value 2lc. Analysis shows that 2lc∼(15–30)× the critical thickness hc. In the case of GaAs on Si (∼4% misfit), 2lc∼40 nm. In material systems such as GaN on Si (∼20% misfit), where the misfit is much larger and interfacial defects are unavoidable, the nanoheteroepitaxial structure is shown to reduce the formation and propagation of threading defects. Nanostructured substrate parameters that impact growth are discussed and interferometric lithography is introduced as a method for fabrication of large-area substrates for nanoheteroepitaxy. Si nanoisland diameters as small as 20 nm are demonstrated. Scanning and transmission electron microscopy data of GaN grown on Si (via organometallic vapor phase epitaxy) shows reduced threading defects in nanostructured samples compared to growth o...

The role of the low temperature buffer layer and layer thickness in the optimization of OMVPE growth of GaN on sapphire

In agreement with previous work,12 a thin, low temperature GaN buffer layer, that is used to initiate OMVPE growth of GaN growth on sapphire, is shown to play a critical role in determining the surface morphology of the main GaN epilayer. X-ray analysis shows that the mosaicity of the main GaN epilayer continues to improve even after several μm of epitaxy. This continuing improvement in crystal perfection correlates with an improvement in Hall mobility for thicker samples. So far, we have obtained a maximum mobility of 600 cm2/V-s in a 6 μm GaN epilayer. Atomic force microscopy (AFM) analysis of the buffer layer and x-ray analysis of the main epilayer lead us to conclude that the both of these effects reflect the degree of coherence in the main GaN epitaxial layer. These results are consistent with the growth model presented by Hiramatsu et al., however, our AFM data indicates that for GaN buffer layers partial coherence can be achieved during the low temperature growth stage.

Electron cyclotron resonance etching characteristics of GaN in SiCl4/Ar

Electron cyclotron resonance (ECR) plasma etching characteristics of gallium nitride (GaN) are investigated using low pressure (4–10 mTorr) SiCl4/Ar ECR discharges. The purpose of this effort is to examine the dry etching processes of GaN that do not require hydrogen, which is known to cause carrier compensation in GaN. A maximum etching rate of 960 A/min and good surface morphologies are obtained. The etch rate is found to increase near‐linearly with increasing dc bias, and a minimum dc bias of 100 V is required to initiate etching. Enhanced etching rates are obtained as the fraction of active chemical etchant species (SiCl4) in the discharge is increased. 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.

EXPERIMENTAL AND THEORETICAL ANALYSIS OF SHADOW-MASKED GROWTH USING ORGANOMETALLIC VAPOR-PHASE EPITAXY : THE REASON FOR THE ABSENCE OF FACETTING

Shadow‐masked growth using organometallic vapor‐phase epitaxy allows the creation of nonplanar regions having smoothly varying profiles devoid of macroscopic facets. The shape of these profiles is independent of gas flow direction and stripe orientation, suggesting that they are determined by lateral gas phase gradients. It is proposed that the absence of facets is due to the unique growth regime underneath the shadow mask, where organic radicals combine with group‐III surface species, temporarily returning them to the vapor phase. This process has been simulated using a two‐dimensional finite‐element analysis that accurately describes the experimental behavior. Furthermore, this hypothesis predicts other experimentally observed phenomena.

Photoelectrochemical etching measurement of defect density in GaN grown by nanoheteroepitaxy

The density of dislocations in n-type GaN was measured by photoelectrochemical etching. A 10× reduction in dislocation density was observed compared to planar GaN grown at the same time. Cross-sectional transmission electron microscopy studies indicate that defect reduction is due to the mutual cancellation of dislocations with equal and opposite Burger’s vectors. The nanoheteroepitaxy sample exhibited significantly higher photoluminescence intensity and higher electron mobility than the planar reference sample.