Michelle T. Leonard

Growth of GaN, InGaN, and AlGaN films and quantum well structures by molecular beam epitaxy

GaN, AIGaN and InGaN films have been grown by molecular beam epitaxy (MBE) using RF plasma sources for the generation of active nitrogen. These films have been deposited homoepitaxially onto GaN/SiC substrates and hetero-epitaxially onto LiGaO 2 substrates. LiGaO 2 is an ordered and closely-lattice-matched orthorhombic variant of the wurtzite crystal structure of GaN. A low-temperature AIN buffer layer is necessary in order to nucleate GaN on LiGaO2. Thick GaN and AIGaN layers may then be grown once deposition is initiated. InGaN has been grown by MBE at mole fractions of up to 20% as a quantum well between GaN cladding layers. The indium containing structures were deposited onto GaN/SiC substrates to focus the development effort on the InGaN growth process rather than on heteroepitaxial nucleation. A modulated beam technique, with alternating short periods of (In, Ga)N and (Ga)N, was used to grow high-quality InGaN. The modulated beam limits the nucleation of metal droplets on the growth surface, which form due to thermodynamic limitations. A narrow PL dominated by band edge luminescence at 421 nm results from this growth technique. Growth of GaN at high temperatures is also reported.

MBE growth of III-V nitride films and quantum well structures using multiple rf plasma sources

MBE growth of III-V nitrides is being studied at NCSU using MOVPE grown GaN buffer layers on SiC as substrates. Rf plasma sources are being used for the generation of active nitrogen during MBE deposition. Through the use of multiple rf plasma sources, sufficient active nitrogen is generated in order to examine the properties of III-V nitride layers grown at higher substrate temperatures and growth rates. The resulting MBE-grown GaN films exhibit remarkably intense photoluminescence (PL) dominated by a sharp band-edge peak at 3,409 eV having a FWHM of 36 meV at 300K. No deep level emission is observed. AlGaN and InGaN films and quantum well structures have also been prepared using multiple sources. A modulated beam MBE approach is used in conjunction with the multiple rf plasma sources to grow InGaN. RHEED and TEM studies reveal flat 2D InGaN quantum well structures. Depending on the indium content, GaN/InGaN single-quantum-well structures exhibit electroluminescence at 300K peaked in the blue-violet to the green spectral region.

Nitride-based emitters on SiC substrates

Single crystal thin films with compositions from the AlN- InN-GaN system were grown via metal-organic chemical vapor deposition on single crystal 6H-SiC substrates. AlGaN containing high and low fractions of Al was grown directly on the SiC for use as a buffer layer. Subsequent epitaxial layers of GaN and AlGaN were doped with Mg and Si to achieve p-type conductivity, respectively. N-type InGaN layers with In compositions up to approximately 50 percent were also achieved. Room temperature photoluminescence on these films exhibited single peaks in the spectral range from the UV to green. Various layers were combined to form light emitting diode (LED) and laser structures. Blue LEDs with both insulating and conductive buffer layers exhibited an external quantum efficiency of 2-3 percent with a forward operating voltage of 3.4-3.7 V. Laser diode structures having a separate confinement heterostructure multiple quantum well configuration were optically and electrically pumped. Photopumping resulted in stimulated emission at 391 nm. Electrically pumped structures resulted in a peak emission at 393 nm and a bandwidth of 12 nm. No lasing was observed.

Recent progress in GaN/SiC LEDs and photopumped lasers

Stimulated emission from optically pumped GaN layers grown on SiC has been reported over a wide temperature range of 77 to 450 K. However, there has been no report of photopumped lasing in a DH structure grown on SiC. This presentation will discuss recent progress in the development of GaN-based blue LEDs at Cree and photopumped lasing results obtained on GaN-AlGaN DH laser structures fabricated on SiC.

Recent progress in AlGaN/GaN laser structures on 6H-SiC

Room temperature hole concentrations of 5 multiplied by 1017 cm-3 and mobilities of 8.4 cm2/V-s have been measured on heavily Mg doped GaN layers grown on SiC. Specific contact resistivities of 0.046 (Omega) -cm2 have been obtained from TLM measurements on ohmic contacts to these layers. Double heterostructures (DH) of GaN/AlxGa1-xN with x equals 0.1 have been grown on n-type 6H-SiC substrates. High quality facets have been fabricated by cleaving these DH structures. Photopumped stimulated emission has been observed in undoped structures at 372 nm at a threshold power density of 72 kW/cm2. An optical gain of 1000 cm-1 was measured in the same samples at 200 kW/cm2.

Chapter 7 SiC-Based UV Photodiodes and Light-Emitting Diodes

Publisher Summary This chapter discusses silicon carbide or Sic-based UV photodiodes and light-emitting diodes (LEDs). Sic has been investigated as a short-wavelength optoelectronic material since the early years of semiconductor development. As a result of the 3.0-eV bandgap of 6H.SiC, any color LED in the visible spectrum can be achieved in this material. Development of 6H-SiC blue LEDs was for many years a major research effort at Siemens Research Laboratory, various labs in the former Soviet Union, and a number of Japanese labs, in particular Sanyo Electric Co., Ltd. Several of these groups released blue LEDs as products. High-temperature Sic ultra violet (UV) photodiodes may have a significant impact in many application areas. Improvement in combustion control is anticipated with the ability to sense flames in aircraft engines, building boiler systems, and industrial processes. Air quality monitoring and UV dosimetry for industrial processes are other important application areas. However, 6H-Sic is an excellent substrate for heteroepitaxial growth of III-nitrides, and Crees line of G. SiC LEDs is based on nitride structures grown on 6H-Sic.

InGaN/GaN lasers grown on SiC

Single crystal thin films with compositions from the AlN-InN- GaN system were grown via metal-organic chemical vapor deposition (MOCVD) on single crystal 6H-SiC substrates. Blue light emitting (LED) and laser diode (LD) structures were fabricated. A conducting buffer layer was developed which uses an AlGaN buffer layer which provides a conduction path between SiC and the active device region. This conducting buffer layer was utilized in both the LEDs and the LDs. The external quantum efficiency of the LEDs was 3% at 20 mA (3.6V) with a peak emission wavelength of 430 nm. Violet and blue LDs were fabricated which consisted of an 8-well InGaN/GaN multiple quantum well (MQW) active region in a separate confinement heterostructure (SCH) design. The devices lased at room temperature under pulsed and continuous wave operation with an emission wavelength of 404-435 nm. The lowest pulsed operation threshold current density obtained for lasing under was 10.4 kA/cm2.

GaN homojunction and AlGaN/GaN heterojunction visible-blind photodiodes grown on SiC

GaN homojunction and AlGaN/GaN heterojunction UV photodiodes were successfully fabricated and tested. The p+/n mesa devices were grown on a n-type 6H-SiC substrate. Photoresponse was observed in these deices from 206 nm to the cutoff wavelength of GaN. Peak responsivity values of 111 mA/W and 123 mA/W were observed at 360 nm for unpackaged homojunction and heterojunction devices, respectively. In packaged device, the peak responsivity increased to 124 and 147 mA/W for the homojunction and heterojunction devices, respectively. High breakdown voltages in excess of 100 V for the homojunction and 70 V for the heterojunction devices were obtained with dark current densities of 3 by 10-11 A/cm2 and 1 by 10-10 A/cm2 A/cm2 at -1V bias at room temperature, respectively. These result show that homojunction and heterojunction visible-blind detectors can be fabricated in the AlGaN/GaN material system on SiC substrates.