A. Alam

Metalorganic Chemical Vapor Phase Epitaxy of Crack-Free GaN on Si (111) Exceeding 1 µm in Thickness

We present a simple method for the elimination of cracks in GaN layers grown on Si (111). Cracking of GaN on Si usually occurs due to large lattice and thermal mismatch of GaN and Si when layer thicknesses exceeds approximately 1 µm. By introducing thin, low-temperature AlN interlayers, we could significantly reduce the crack density of the GaN layer. The crack density is practically reduced to zero from an original crack density of 240 mm-2 corresponding to crack-free regions of 3×10-3 mm2. Additionally for the GaN layer with low temperature interlayers, the full width at half maximum X-ray (2024) rocking curve is improved from approximately 270 to 65 arcsec.

Bright blue electroluminescence from an InGaN/GaN multiquantum-well diode on Si(111): Impact of an AlGaN/GaN multilayer

We present an electroluminescence test structure which consists of an InGaN/GaN multiquantum well as active region on the top of an AlGaN/GaN multilayer grown by metalorganic vapor phase epitaxy on Si(111) substrate. The integral room-temperature electroluminescence spectrum reveals a peak emission wavelength of 467 nm and a significantly higher brightness than an identical reference structure on sapphire substrate. In microelectroluminescence imaging, two emission peaks at 465 and 476 nm can be separated originating from locally different areas of the diode. Cathodoluminescence measurements in cross section and high-resolution x-ray diffraction measurements show that the structure is less strained than a sample without the AlGaN/GaN multilayer. The AlGaN/GaN multiple layer sequence which has a total thickness of 1.5 μm causes lattice relaxation during growth after a thickness of around 0.9 μm as directly visualized by cathodoluminescence line scans across the diode.

Crack‐Free InGaN/GaN Light Emitters on Si(111)

We present MOCVD-grown crack-free GaN based light emitters on Si with a layer thickness of 3.6 μm. The crack free layer is grown on a thin predeposited GaN layer with fields defined by a Si x N y mask. In X-ray diffraction measurements a reduction in stress is observed for the patterned sample as compared to a similar unstructured sample. Lateral growth occurs at the edges of the fields with strongly differing growth rates for perpendicular directions. The impact of the facet type on the growth rate and impurity incorporation is observed by scanning electron microscopy and cathodoluminescence measurements. In electroluminescence the diode shows a bright blue emission at 421 nm.

Luminescence and Stimulated Emission from GaN on Silicon Substrates Heterostructures

Photoluminescence (PL) and stimulated emission of GaN/Si layers grown by MOVPE with AIN and AlGaN buffers have been investigated. It has been found that thermal annealing of samples in nitrogen gas flow leads to an increase of the PL efficiency of up to 20 times, with a thermal activation energy of this process of 3.1-3.4 eV. The annealing promotes a significant decrease of the non-radiative center concentration and an increase of the number of shallow states. The laser action at λ = 377 nm under pulsed optical excitation was achieved at room temperature evidencing a high quality of the samples.

GaN-on-Si Enhancement Mode Metal Insulator Semiconductor Heterostructure Field Effect Transistor with On-Current of 1.35 A/mm

GaN-on-Si transistors are regarded as a candidate for future power-switching applications. Beside the necessity to achieve enhancement mode behavior, on-resistance and maximum gate voltage are still limited for GaN-based transistors on Si substrate. Here, an enhancement mode metal insulator semiconductor heterostructure field effect transistor on Si substrate with record on-current of 1.35 A/mm and threshold voltage of +0.82 V is demonstrated. The corresponding gate current is still well below 1 mA/mm at 6.5 V gate voltage. By comparison of measured and simulated CV curves, the density of interface states introduced by the insulator is shown to be quasi-independent on etch damage and/or barrier material.

Nearly strain-free AlGaN on (0 0 0 1) sapphire : X-ray measurements and a new crystallographic growth model

Two micrometer thick Al x Ga 1-x N layers with 0 < x < 0.4 were grown by low-pressure metal organic chemical vapour deposition on sapphire (0001) substrates. For Al concentrations 0.18 < x < 0.25 the layers are found to be nearly strain-free as determined by high resolution X-ray reciprocal space mapping around the (0002), (20 - 24), and (20 - 20) Bragg reflections in conventional and grazing incidence geometry, respectively. The in-plane lattice parameter a of layers grown in this composition regime coincides with that of (2/3)a (sapphire) . Their rotational and tilting disorder shows a minimum as compared to layers grown outside this regime. (GaN/Al x Ga 1 -x N) multi-quantum well structures on top of such buffer layers are fully pseudomorphic having lowest interface disorder and best surface morphology as evaluated by specular and diffuse X-ray reflectivity measurements. The findings are explained by the assumption of a 2D coincidence site lattice for the epitaxial growth of AlGaN on sapphire. The coincidence site lattice has hexagonal symmetry with the lattice parameter three times a(A 0.22 Ga 0.78 N) equals two times a(sapphire).

Luminescence and lasing in InGaN∕GaN multiple quantum well heterostructures grown at different temperatures

It was found that the decrease of the InGaN∕GaN multiple quantum well (MQW) growth temperature from 865 to 810 °C leads to a MQW emission wavelength shift from the violet to the green spectral region. The lowering of the growth temperature also promotes a decrease of the MQW photoluminescence (PL) intensity at high excitation and a disappearance of the excitonic features from the low-temperature reflection and PL spectra of GaN barriers and claddings. The laser threshold dependence on Tg is not monotonic, with the lowest value of 270kW∕cm2 at Tg=830°C. High-temperature annealing (900 °C, 30 min) leads to a twofold increase of the PL efficiency only from the InGaN QWs grown at the lowest temperature. The results allow one to explain the laser threshold behavior in terms of the heterostructure quality, the defect concentration, In clusterization, and the piezoelectric field dependence on the MQW growth temperature.

Investigation of plasma-oxidized aluminium as a gate dielectric for AlGaN/GaN MISHFETs

Nitride-based metal insulator semiconductor heterostructure field effect transistors (MISHFETs) have shown great potential, recently. An elegant technological approach is presented to fabricate MISHFETs with the gate dielectric formed only in the gate region. Aluminium is deposited in the gate region, oxidized by an inductively coupled oxygen plasma, subsequently covered by the evaporated gate contact metal and finally lifted off, all using a single patterning step. The electrical properties of the dielectric fabricated in this manner can be enhanced by annealing. Leakage currents comparable to those of other dielectric deposition methods are achieved. A subthreshold swing below 80 mV dec−1 indicates a high-quality interface.

Spectral-angular and threshold characteristics of ultraviolet-blue In(Al)GaN/GaN/Al2O3 heterostructure lasers

The influence of layer thickness, heterostructure design, optical confinement factor and spontaneous emission efficiency on laser parameters of GaN based quantum well optically pumped lasers is studied in wide spectral (373 - 470 nm), temperature (77 - 600 K) and excitation intensity (102 - 3 106 W/cm2) regions. The laser threshold enhancement from 70 kW/cm2 for the 421 nm operating laser to 900 kW/cm2 for the 469.5 nm laser leads to the reduction of highest operation temperature of the laser from 585 K for the 421 nm laser to 295 K for the 469.5 nm laser with increasing operating wavelength. As a rule the far field pattern of the laser emission consists of two light spots localized at positive and negative angles of 30 - 50 degree(s). The laser spectra structure in the far-field of the SQWs and MQWs with low thickness of the active layers depended on the registration angle. The spatial distribution of the laser light in the far-field consisting of transverse and leaky modes was calculated and compared with the experimental results. Calculations of the optical confinement factor and the electromagnetic field distribution inside and outside of the heterostructures showed that the MQW lasers operate in the high order transverse mode regime. The spectral-angular distribution of the emission of the SQW and MQW lasers with low active layer thickness is due to the leaky mode formation.

Silicon as a substrate in multiwafer MOVPE GaN technology

We report results on the transferability of a blue-green electroluminescence test structure (ELT) process across different reactor geometries and substrate materials. The process was transferred from the conditions of our well-known 6 X 2 inch to the 5 X 3 inch AIX 2400 G3 geometry by simple up-scaling of the respective process parameters in accordance with numerical simulations done on the reactor setup. The five period InGaN/GaN quantum well ELT structures with an average emission wavelength on wafer of 480 nm shows a standard deviation of 1 - 2% without rim exclusion. Electroluminescence up to 560 nm were achieved in InGaN/GaN structures with high In content. With these prospects new types of seed layers for the transfer of our standard electroluminescence test structures (ELT) process to Si- substrates were investigated. The growth on different seed layers was found feasible and resulted in operational ELT structures with emission wavelengths in the range of 440 nm to 470 nm. Electrical quick test shows bright blue emission across the full Si wafer.