Electron beam induced current measurements on planar Schottky diodes on undoped GaN grown by metalorganic chemical vapor deposition are reported. The minority carrier diffusion length of 0.28 μm has been measured, indicating minority carrier lifetime of 6.5 ns. The tapping mode atomic force microscopy imaging of the surfaces and scanning electron microscopy of the cross sections have been used to characterize the linear dislocations and columnar structure of the GaN. The possible influence of recombination on the extended defects in GaN on the minority carrier diffusion length and lifetime is discussed, and contrasted to other recombination mechanisms.

We fabricated high standoff voltage ͑450 V͒ Schottky rectifiers on hydride vapor phase epitaxy grown GaN on sapphire substrate. Several Schottky device geometries were investigated, including lateral geometry with rectangular and circular contacts, mesa devices, and Schottky metal field plate overlapping a SiO 2 layer. The best devices were characterized by an ON-state voltage of 4.2 V at a current density of 100 A/cm 2 and a saturation current density of 10 Ϫ5 A/cm 2 at a reverse bias of 100 V. From the measured breakdown voltage we estimated the critical field for electric breakdown in GaN to be (2.2Ϯ0.7)ϫ10 6 V/cm. This value for the critical field is a lower limit since most of the devices exhibited abrupt and premature breakdown associated with corner and edge effects. © 1999 American Institute of Physics. ͓S0003-6951͑99͒02409-2͔Wide band gap materials, primarily SiC and GaN, have recently attracted a lot of interest for applications in high power and high temperature electronics. Although the processing technology for SiC is more mature, GaN offers several advantages. First, there are various device possibilities using GaN/AlGaN heterojunctions which are not available in the SiC system. Second, the availability of cheap and efficient hydride vapor phase epitaxy ͑HVPE͒ growth technology achieving growth rates in excess of 100 m/h, have produced thick, high quality GaN layers on sapphire. 1 Third, by using AlGaN layers, one can take advantage of a larger band gap to achieve higher critical electric fields than in GaN alone.In this study, we focus on the fabrication of high voltage, GaN based Schottky rectifiers and the measurement of the critical field for electric breakdown. The critical field for electric breakdown is one of the most significant parameters in the design and performance of high power devices. It directly influences the required thickness of the standoff region in the Schottky rectifier and bipolar devices, such as the thyristor. Since the thickness of the standoff region sets the resistivity of the device, it will determine power dissipation and maximum current density of the device. 2,3 In previous studies, Schottky diodes have been fabricated on GaN using a variety of elemental metals including Pd and Pt, 4,5 , Au, Cr, and Ni,6,7 , and Mo and W. 8 More details on the metal-GaN contact technology can be found in Ref. 9.In this work, Schottky rectifiers were fabricated on 8-10 m thick GaN layers grown by HVPE on sapphire, where the electron concentration changes with the distance from the GaN/sapphire interface. We carried out conductivity and Hall measurements on a series of HVPE GaN films of varying thickness ranging from 0.07 to 9.2 m, and fitted the data to a two layer model. [10][11][12][13] From the model, we concluded that the GaN films consisted of a low conductivity, low electron concentration, 8-10 m thick top layer on a very thin (Ͻ100 nm), highly conductive, high electron concentration bottom layer. The electron concentrations and mobilities in the thin interface layer and thic...

We have studied molecular beam epitaxy grown GaN films of both polarities using electric force microscopy to detect sub 1 m regions of charge density variations associated with GaN extended defects. The large piezoelectric coefficients of GaN together with strain introduced by crystalline imperfections produce variations in piezoelectrically induced electric fields around these defects. The consequent spatial rearrangement of charges can be detected by electrostatic force microscopy and was found to be on the order of the characteristic Debye length for GaN at our dopant concentration. The electric force microscope signal was also found to be a linear function of the contact potential between the metal coating on the tip and GaN. Electrostatic analysis yielded a surface state density of 9.4Ϯ0.5ϫ1010 cm Ϫ2 at an energy of 30 mV above the valence band indicating that the GaN surface is unpinned in this case. © 1999 American Institute of Physics. ͓S0003-6951͑99͒01223-1͔Nitride based devices have been of great interest in the last few years, notably due to their success in optoelectronics, where lasers and diodes have been demonstrated and successfully commercialized.1 Further applications of nitrides are expected in the arena of high power and high temperature devices, 2-4 as well as solar blind ultraviolet detectors.5 It has been recently demonstrated that the large intrinsic piezoelectric coefficients of GaN and AlN are responsible for a high concentration two-dimensional electron gas at the AlGaN/GaN interface in heterojunction field effect transistors ͑HFET͒.6,7 Other possibilities exist for the enhancement of electric properties of contacts to nitrides by piezoelectric engineering as recently demonstrated in the case of Schottky contacts.8 While most of the recent research has emphasized electronic device aspects of the piezoelectric effect, 6-8 comparatively little work has concentrated on the investigation of fundamental properties and nanoscale characterization of piezoelectrically induced phenomena. One consequence of the piezoelectric effect is that it allows electrostatic force imaging of charge redistribution around defects due to local variations in strain caused by crystalline imperfections. Although the magnitude of the charge density is nonquantitative, electric force microscopy ͑EFM͒ can still provide interesting insight into the nature of defects, the piezoelectric effect in nitrides, as well as measurement of the surface state density and energy. 9,10The gallium nitride layers studied here were grown on c-plane sapphire substrates by radio frequency plasma assisted molecular beam epitaxy. Ga-polar GaN films were nucleated using AlN buffer layers whereas N-polar films were nucleated using a GaN buffer layer. Polarity was determined by reflection high-energy electron diffraction ͑RHEED͒ reconstruction at low temperature, 11 and by ͑KOH͒ etching. 12,13 Other details of the growth conditions are presented elsewhere. 14,15 A variety of different metals were used for coating atomic force microscope ͑AF...

We report on electron beam induced current and current-voltage (I -V) measurements on Schottky diodes on p-type doped GaN layers grown by metal organic chemical vapor deposition. A Schottky barrier height of 0.9 eV was measured for the Ti/Au Schottky contact from the I -V data. A minority carrier diffusion length for electrons of (0.2Ϯ0.05) m was measured for the first time in GaN. This diffusion length corresponds to an electron lifetime of approximately 0.1 ns. We attempted to correlate the measured electron diffusion length and lifetime with several possible recombination mechanisms in GaN and establish connection with electronic and structural properties of GaN. © 1998 American Institute of Physics. ͓S0003-6951͑98͒03848-0͔The wide band gap semiconductors GaN and AlGaN are already established materials for light emitting diodes and lasers, 1 and have recently attracted a lot of interest for applications in high power and high temperature electronics. 2 Some of the most significant parameters influencing the design and performance of both unipolar and bipolar high power devices are the critical field for electric breakdown and the transport parameters, such as minority carrier diffusion lengths and lifetimes, for both holes and electrons. Correlation of these parameters with material properties is important for succesful development of power electronics systems based on the nitrides. Minority carrier diffusion lengths and lifetimes are critical parameters for the performance of thyristor switches and other bipolar devices. 3,4 Larger hole lifetimes in the n-type standoff layer of thyristors lead to smaller ON-state voltages and therefore smaller power dissipation and larger maximum current densities. 3,4 The electron diffusion length is important because it determines the range of suitable thicknesses for the p-type injection layer in bipolar structures. In this study we report a measurement of the electron diffusion length and estimate the electron lifetime as a minority carrier in GaN.We fabricated Schottky diodes on Mg-doped GaN/ sapphire layers grown by metal organic chemical vapor deposition ͑MOCVD͒. The experiments were performed on samples from two commercial vendors. For both samples the hole concentration measured by Hall measurements was approximately (1 -4)ϫ10 17 cm Ϫ3 , and the hole mobility was approximately 5 cm 2 /V s. Prior to metal deposition, the GaN surfaces were cleaned with organic solvents, dipped in HF:H 2 O ͑1:10͒, rinsed in deionized water, then blown dry with nitrogen gas. Contact metals were sputtered in a chamber with background pressure of 2ϫ10 Ϫ8 Torr and patterned to produce Schottky contacts as well as large-area ohmic contacts. A Ni/Au ͑200 Å/1500 Å͒ metallization scheme was used for the ohmic contacts, while Ti/Au was used for the Schottky contacts.Current-voltage characteristics were measured at room temperature with an HP 4156 parameter analyzer and are shown in Fig. 1. From these measurements we obtained a barrier height of ⌽ Bp ϭ0.9 eV. Since the reported barrier heigh...

An electrostatic force microscope was used to write and image localized dots of charge in a double barrier CeO 2 /Si/CeO 2 /Si͑111͒ structure. By applying a relatively large tip voltage and reducing the tip to sample separation to 3-5 nm, charge dots 60-200 nm full width at half maximum of both positive and negative charge have been written. The total stored charge is found to be Q ϭϮ(20-200)e per charge dot. These dots of charge are shown to be stable over periods of time greater than 24 h, with an initial charge decay time constant of ϳ9.5 h followed by a period of much slower decay with Ͼ24 h. The dependence of dot size and total stored charge on various writing parameters such as tip writing bias, tip to sample separation, and write time is examined. © 1999 American Institute of Physics. ͓S0003-6951͑99͒04035-8͔ Cerium oxide (CeO 2 ) is an insulating material with a lattice mismatch of only 0.35% to silicon ͑Si͒ and an energy band gap of ϳ5.5 eV. This attractive set of properties has the potential to lead to a fully functional silicon heterojunction technology. A significant amount of work has been done examining the growth and characterization of CeO 2 crystals on Si, 1-5 and the growth of single crystal Si on to CeO 2 /Si heterostructures 6 has been recently reported. Based on these promising results, a silicon resonant tunneling diode, an improved silicon-on-insulator technology, and stacked silicon electronics have all been proposed. A valuable and interesting addition to this array of technologies would be the capacity for integrated electrostatic data storage.In this letter, we describe the localized charging and subsequent imaging of a double barrier CeO 2 /Si/CeO 2 /Si͑111͒ structure by electrostatic force microscopy ͑EFM͒.7-9 The controllable writing of both positive and negative localized dots of charge with long lifetimes is described and it is further shown that these charge dots may be rewritten and replaced by charge of the opposite sign through the application of an opposite charging bias. A simple analysis is presented to quantify the total amount of charge stored in each charge dot. The time evolution of these charge dots is studied, and charge decay time constants are extracted. Finally, a study is presented of various writing parameters such as tip bias, tip to sample separation, and write time on the size and total stored charge of the resultant charge dots.Samples were produced from commercially available 3 in. Si͑111͒ wafers, n-type with 3.0-4.3 ⍀ cm resistivity. After being subjected to a standard acetone, isopropyl alcohol, deionized water degrease in ultrasound, the wafer was etched in 50:1 HF solution until hydrophobic, rinsed in deionized water, and immediately introduced into vacuum. Electron beam evaporation was used to deposit material from an undoped Si charge and a 99.99% CeO 2 charge to grow the structures. Initially, a 200 Å Si buffer layer was grown and examined by ͑RHEED͒ reflection high-energy electron diffraction to assure the characteristic (7ϫ7) reconstruction was appar...