J. W. Graff

Low-resistance ohmic contacts to p-type GaN

The specific contact resistance of two types of ohmic contacts to p-type GaN is analyzed. First, an ohmic contact formed by a metal electrode deposited on a highly doped p-type GaN layer. Second, an ohmic contact formed by a metal electrode deposited on a thin GaN layer with an internal electric field caused by polarization effects. It is shown that contacts mediated by polarization effects can result, for typical materials parameters, in low contact resistances comparable or better than contacts mediated by dopant-induced surface fields. A type of contact is proposed and demonstrated. These contacts employ polarization charges to enhance tunneling transport as well as high doping. Experimental results on Ni contacts to p-type AlxGa1−xN/GaN doped superlattices are presented. The contacts have linear current–voltage characteristics with contact resistances of 9.3×10−4 Ω cm2, as inferred from linear transmission-line method measurements. The influence of annealing at temperatures ranging from 400 to 500 °C ...

Improved mobilities and resistivities in modulation-doped p-type AlGaN/GaN superlattices

The transport properties of modulation, shifted modulation, and uniformly doped Al0.20Ga0.80N/GaN superlattices are presented. The modulation-doped sample is doped only in the AlGaN barriers. The shifted-modulation-doped sample has its dopants shifted by one-quarter period. Measurements reveal a strong improvement in mobility and resistivity for the modulation-doped and shifted-modulation-doped structures versus the uniformly doped structure. The modulation-doped sample has a mobility of 9.2 and 36 cm2/V s at 300 and 90 K respectively and a very low resistivity of 0.20 and 0.068 Ω cm at 300 and 90 K, respectively. Capacitance–voltage profiling shows multiple two-dimensional hole gases. The results are consistent with a reduction of neutral impurity scattering for modulation-doped structures as compared to uniformly doped structures.

Ohmic contacts to p-type GaN mediated by polarization fields in thin InxGa1−xN capping layers

Low-resistance ohmic contacts are demonstrated using thin p-type InGaN layers on p-type GaN. It is shown that the tunneling barrier width is drastically reduced by polarization-induced electric fields in the strained InGaN capping layers resulting in an increase of the hole tunneling probability through the barrier and a significant decrease of the specific contact resistance. The specific contact resistance of Ni (10 nm)/Au (30 nm) contacts deposited on the InGaN capping layers was determined by the transmission line method. Specific contact resistances of 1.2×10−2 Ω cm2 and 6×10−3 Ω cm2 were obtained for capping layer thicknesses of 20 nm and 2 nm, respectively.Low-resistance ohmic contacts are demonstrated using thin p-type InGaN layers on p-type GaN. It is shown that the tunneling barrier width is drastically reduced by polarization-induced electric fields in the strained InGaN capping layers resulting in an increase of the hole tunneling probability through the barrier and a significant decrease of the specific contact resistance. The specific contact resistance of Ni (10 nm)/Au (30 nm) contacts deposited on the InGaN capping layers was determined by the transmission line method. Specific contact resistances of 1.2×10−2 Ω cm2 and 6×10−3 Ω cm2 were obtained for capping layer thicknesses of 20 nm and 2 nm, respectively.

Photon recycling semiconductor light-emitting diode

A new white light emitting diode, the photon recycling semiconductor light emitting diode (PRS-LED) is demonstrated. The device consists of a GaInN/GaN LED emitting in the blue spectral range and an AlGaInP photon recycling semiconductor emitting at the complementary color. The PRS-LED thus has two emission lines, one in the blue and one in the amber wavelength range. The theoretical luminous efficiency of the PRS-LED exceeds 300 lm/W, higher than the efficiency of phosphor-based white LEDs.

GaInN light-emitting diodes with omni directional reflectors

A high-reflectivity omni directional reflector (ODR) has been incorporated into a GaInN light-emitting diode (LED) structure. The ODR comprises a transparent, electrically conductive quarter-wave layer of indium tin oxide clad by silver and serves as an ohmic contact to p-type GaN. It is shown that ODR-LEDs have low optical losses and high extraction efficiency. Mesa-structure GaInN/GaN ODR-LEDs emitting in the blue wavelength range are demonstrated and compared to GaInN/GaN LEDs with semitransparent Ni/Au top contacts. The extraction efficiency of ODR-LEDs is higher as compared to conventional LEDs with Ni/Au contacts.

Ohmic contact technology in III nitrides using polarization effects of cap layers

A technology for low-resistance ohmic contacts to III nitrides is presented. The contacts employ polarization-induced electric fields in strained cap layers grown on lattice-mismatched III-nitride buffer layers. With appropriate choice of the cap layer, the electric field in the cap layer reduces the thickness of the tunnel barrier at the metal contact/semiconductor interface. Design rules for polarization-enhanced contacts are presented giving guidance for composition and thickness of the cap layer for different III-nitride buffer layers. Experimental results for ohmic contacts with p-type InGaN and GaN cap layers are markedly different from samples without a polarized cap layer thus confirming the effectiveness of polarization-enhanced ohmic contacts.

Experimental study of perpendicular transport in weakly coupled AlxGa1−xN/GaN superlattices

Perpendicular transport characteristics of n-type AlxGa1−xN/GaN superlattices are presented. Planar and mesa-etched superlattice structures are employed to identify the perpendicular resistance. Perpendicular transport measurements in Al0.22Ga0.78N/GaN superlattices display linear current–voltage characteristics with a resistivity that is a factor of 6.6 higher than for bulk material. A theoretical model is developed for perpendicular transport in AlxGa1−xN/GaN superlattices based on sequential tunneling. The model shows that short superlattice periods are required to minimize the perpendicular resistivity.

Influence of electron injection on performance of GaN photodetectors

It is demonstrated that short-time (up to 1200 s) electron injection into the p-region of GaN p–n junction, as a result of forward bias application, leads to a long-term multifold enhancement of the device peak responsivity as well as to a spectral broadening of the photoresponse. The effect is found to persist for several days and is related to an increased minority carrier diffusion length in the p region, due to an injected electron trapping on deep levels associated with Mg acceptors.

Minority electron transport anisotropy in p-type Al/sub x/Ga/sub 1-x/N/GaN superlattices

The minority electron diffusion length, L, in Mg-doped molecular beam epitaxy (MBE) grown p-type Al/sub x/Ga/sub 1-x/N/GaN superlattices with aluminum content x=0.1 and 0.2 was measured perpendicular and parallel to the superlattice planes by the electron beam induced current technique. A large anisotropy in the transport properties was observed with the effect varying from 1:3 to 1:6. We attribute an experimentally observed diffusion length anisotropy to minority electron scattering during transport across the potential barriers of the superlattice. Reference p-GaN samples were also investigated, and the diffusion length was observed to be isotropic in both MOCVD (L=0.5 /spl mu/m) and MBE (L=0.27 /spl mu/m) grown samples.

AlGaInP light-emitting diodes with omnidirectionally reflecting submount

A novel AlGaInP light-emitting diode (LED) is presented that employs high-reflectivity omni-directional reflector (ODR) submounts. It is shown that the reflective-submount (RS) LED has a higher light-extraction efficiency than conventional LEDs. Red AlGaInP RS-LEDs bonded to Si-substrates are demonstrated using a silver-based ODR. The ODR is perforated by an array of small-area low-resistance ohmic contacts. The optical and electrical characteristics of the RS-LEDs are presented and compared to conventional AlGaInP absorbing substrates (AS) LEDs with distributed Bragg reflectors (DBR). It is shown that the light output from the RS-LED exceeds that of AS-LEDs by about a factor of two.