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  • H01S5/34333—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser with a well layer based on Ga(In)N or Ga(In)P, e.g. blue laser
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  • H10P14/32—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials characterised by intermediate layers between substrates and deposited layers
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  • H10P14/3242—Structure
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  • H10P14/3414—Deposited materials, e.g. layers characterised by the chemical composition being group IIIA-VIA materials
  • H10P14/3416—Nitrides
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  • H01S2304/04—MOCVD or MOVPE
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  • H01S2304/00—Special growth methods for semiconductor lasers
  • H01S2304/12—Pendeo epitaxial lateral overgrowth [ELOG], e.g. for growing GaN based blue laser diodes
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  • H01S5/00—Semiconductor lasers
  • H01S5/02—Structural details or components not essential to laser action
  • H01S5/0206—Substrates, e.g. growth, shape, material, removal or bonding
  • H01S5/0213—Sapphire, quartz or diamond based substrates

Definitions

• This invention is related to compound semiconductor growth and device fabrication. More particularly the invention relates to the growth and fabrication of indium gallium nitride (InGaN) containing electronic and optoelectronic devices by metalorganic chemical vapor deposition (MOCVD).
• InGaN indium gallium nitride
• MOCVD metalorganic chemical vapor deposition
• MBE molecular beam epitaxy
• MOCVD metalorganic chemical vapor deposition
• HVPE hydride vapor phase epitaxy
• FIG. 1 is a schematic of a generic hexagonal wurtzite crystal structure 100 and planes of interest 102, 104, 106, 108 with these axes 110, 112, 114, 116 identified therein, wherein the fill patterns are intended to illustrate the planes of interest 102, 104 and 106, but do not represent the materials of the structure 100.
• Group III and nitrogen atoms occupy alternating c-planes along the crystal's c-axis.
• the symmetry elements included in the wurtzite structure dictate that Ill-nitrides possess a bulk spontaneous polarization along this c-axis.
• wurtzite nitrides can and do additionally exhibit piezoelectric polarization, also along the crystal's c-axis.
• Current nitride technology for electronic and optoelectronic devices employs nitride films grown along the polar c-direction.
• conventional c-plane quantum well structures in Ill-nitride based optoelectronic and electronic devices suffer from the undesirable quantum-confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations.
• QCSE quantum-confined Stark effect
• (Al,Ga,In)N quantum-well structures employing nonpolar growth directions, e.g., the l 12 ⁇ ⁇ -direction or (l 1 O ⁇ w-direction, provide an effective means of eliminating polarization-induced electric field effects in wurtzite nitride structures since the polar axis lies within the growth plane of the film, and thus parallel to heterointerfaces of quantum wells.
• growth of nonpolar growth directions e.g., the l 12 ⁇ ⁇ -direction or (l 1 O ⁇ w-direction
• nonpolar nitrides are typically grown above 900°C and more often above 1050°C, temperatures at which In readily desorbs from the surface.
• high-quality nonpolar nitrides are typically grown at decreased pressures ( ⁇ 100 Torr) in order to stabilize the a- and m-planes relative to inclined facets.
• c- plane InGaN should be grown at atmospheric pressure in order to enhance In incorporation and decrease carbon incorporation.
• the present invention overcomes these challenges and for the first time yields high quality InGaN films and InGaN-containing devices by MOCVD.
• the present invention describes a method for fabricating high- quality indium (In) containing epitaxial layers and heterostructures and devices, including planar nonpolar InGaN films.
• the method uses MOCVD to realize nonpolar InGaN/GaN violet and near-ultraviolet light emitting diodes and laser diodes.
• FIG. 1 illustrates a hexagonal wurtzite crystal structure with its axes identified.
• FIG. 2 is a flowchart describing the process steps according to the preferred embodiment of the present invention.
• FIG. 3 is a schematic cross-section of the nonpolar light emitting diode.
• FIG. 4 is a graph of the current- voltage (I-V) characteristic of the nonpolar
• FIG. 5 is a graph of the electroluminescence (EL) spectra for different driving currents, wherein the inset shows the EL linewidth as a function of the driving current.
• FIG. 6 is a graph of the on-wafer output power and external-quantum efficiency (EQE) of the LED as a function of the drive current.
• EQE external-quantum efficiency
• Nonpolar nitride semiconductors offers a means of eliminating polarization effects in wtirtzite-structure IILnitride devices.
• Current (Ga,Al,In,B)N devices are grown in the polar [0001] c-direction, which results in charge separation across heterostructures.
• the resulting polarization fields are detrimental to the performance of current state of the art devices, particularly for optoelectronic devices. Growth of such devices along a nonpolar direction could significantly improve device performance.
• the present invention now allows the fabrication of nonpolar InGaN films as well as nonpolar InGaN-containing device structures. Previous problems related to gross surface roughening, low In incorporation, and In desorption in InGaN heterostructures have been overcome by this technique.
• This MOCVD-based invention has been applied to the realization of the first nonpolar InGaN/GaN violet LEDs. This invention enables the production of nonpolar GaN-based visible and near-ultraviolet LEDs and LDs for the first time.
• the present invention is an approach for fabrication of high-quality In- containing epitaxial layers and heterostructures and devices containing the same.
• Superior planar nonpolar InGaN films have been grown by MOCVD, and functional nonpolar InGaN-containing devices have been fabricated by the same technique.
• This particular demonstration involves the fabrication of ⁇ -plane oriented InGaN-based quantum wells
• research on m-plane nitride growth has indicated that the techniques described herein are broadly applicable to the growth of w-plane InGaN/GaN devices as well.
• Planar nonpolar ⁇ -plane GaN templates were grown by MOCVD. The details of the template growth are disclosed in co-pending and commonly- assigned Patent Application Nos.
• the ⁇ -plane GaN template on r- plane sapphire substrate is grown by a two-step process which includes a low temperature (620 - 650° C) GaN nucleation layer step and a high temperature (1130 — 1180° C) GaN growth step. A V/III ratio between 650 and 670 is used.
• the GaN growth rate measured by in-situ thickness measurement using reflectance spectroscopy, is in the range 4-6 A/s.
• a total flow of 10 slpm is employed during the ULD GaN growth.
• the above growth procedure established the feasibility of growing nonpolar InGaN.
• the present invention is directed to the growth and fabrication of a nonpolar InGaN-based LED.
• Block 200 represents providing a smooth, low-defect-density Ill-nitride substrate or template.
• this Block may represent the fabrication, on an r- plane sapphire substrate 300, of a 10 ⁇ m-thick reduced-dislocation-density lateral epitaxial overgrown (LEO) ⁇ -plane GaN template 302 by HVPE.
• LEO reduced-dislocation-density lateral epitaxial overgrown
• PCTUS03/21918 (30794.93-WO-U1), which is set forth above and incorporated by reference herein.
• the template 300 is GaN, it could also comprise aluminum nitride (A1N) or aluminum gallium nitride (AlGaN).
• AlGaN aluminum gallium nitride
• m-plane GaN templates could be fabricated as well.
• the mask for the LEO process comprises parallel 8 ⁇ m wide SiO stripes separated by 2 ⁇ m wide window openings oriented parallel to the GaN ⁇ 1 1 00> direction.
• Block 202 represents the re-growth, carried out in a vertical MOCVD reactor, which begins with a 2.2 ⁇ m Si doped «-GaN base layer 304 with an electron concentration of 2 x 10 cm " .
• This layer is deposited under typical ⁇ -plane GaN growth conditions (e.g., substrate temperature 1050-1150°C, system pressure 40-100 Torr, H 2 carrier gas, V/ffl ⁇ 100).
• substrate temperature 1050-1150°C substrate temperature 1050-1150°C
• system pressure 40-100 Torr system pressure 40-100 Torr
• H 2 carrier gas H 2 carrier gas
• V/ffl ⁇ 100 substrate temperature 1050-1150°C
• a substrate that comprises a planar nonpolar ⁇ -plane GaN template grown by MOCVD.
• a smooth, low-defect-density Ill-nitride substrate may be provided.
• Such substrates may include a low-defect-density free-standing a-plane GaN wafer, a low-defect-density free-standing m-plane GaN wafer, a low-defect- density free-standing a-plane A1N wafer, a low-defect-density free-standing m-plane A1N wafer, a low-defect-density bulk a-plane GaN wafer, a low-defect-density bulk m-plane GaN wafer, a low-defect-density bulk a-plane A1N wafer, or a low-defect- density bulk m-plane A1N wafer.
• Block 204 represents the deposition of an InGaN/GaN active region 306 for the device at a reduced temperature, at atmospheric pressure, using N 2 carrier gas.
• This Block includes: (1) growing nonpolar InGaN layers on the substrate or template at a reduced temperature (near or at approximately 900°C) using an N carrier gas to enhance In incorporation and decrease In desorption, wherein the InGaN layers are grown near or at atmospheric pressure (near or at approximately 760 Torr) to enhance InGaN film quality and decrease carbon incorporation, (2) growing a thin low- temperature GaN capping layer on the nonpolar InGaN layers to prevent In desorption during the later growth of a p-type GaN layer, and (3) growing one or more
• the active region 306 is comprised of a 5 period MQW stack with . 16 nm Si-doped GaN barriers and 4 nm InQ. ⁇ 7 Gao. 83 N quantum wells.
• Block 206 represents growing an undoped GaN barrier 308 near or at atmospheric pressure on the InGaN/GaN MQW structure 306. Specifically, this Block represents the deposition of a 16 nm undoped (or unintentionally doped (ULD)) GaN barrier 308 at low temperature to cap the InGaN MQW structure 306 in order to prevent desorption of InGaN from the active region 306 later in the growth.
• ULD unintentionally doped
• Block 208 represents growing one or more n-type and p-type (Al,Ga)N layers 310 at low pressure (near or at approximately 20 - 150 Torr) on the undoped GaN barrier 308.
• this Block represents the deposition of a 0.3 ⁇ m Mg-doped p-type GaN layer 310 with a hole concentration of 6 x 10 17 cm “3 at a higher temperature ( ⁇ 1100°C) and lower pressure (-70 Torr), wherein a total flow of 16 slpm is employed for the p-type GaN growth.
• Block 210 represents the deposition of a 40 nm heavily doped ? + -GaN layer
• Block 212 represents the deposition of a Pd/Au contact 314 and an Al Au contact 316, as/?-GaN and n-GaN contacts respectively, for the device.
• the end result of these process steps is a nonpolar InGaN based heterostructure and device. Specifically, the end result of these process steps is an InGaN LED or LD.
• Relative optical power measurements under direct current (DC) conditions were obtained from the backside emission through the sapphire substrate onto a calibrated broad area Si photodiode.
• the emission spectrum and the optical power emission of the LEDs were measured as a function of driving currents, as shown in FIGS. 5 and 6, respectively. All measurements were carried out at room temperature.
• the device structure described above constitutes the first report of a functioning InGaN-based LED.
• the I- V curve (FIG. 4) of the diode exhibited a forward voltage of 3.3 V with a low series resistance of 7.8 ⁇ .
• Nonpolar ⁇ -plane GaN p-n junction diodes grown under identical conditions on planar a-plane GaN templates exhibited similar forward voltage but had higher series resistances on the order of 30 ⁇ .
• the lower series resistance in these LEDs can be attributed to the higher conductivity in the defect free overgrown region of the LEO GaN template.
• the electroluminescence (EL) spectra of the devices were studied as a function of the dc driving current. Emission spectra were measured at drive currents ranging from 10 to 250 mA.
• the PL spectra on the as-grown sample showed a strong quantum- well emission at 412 nm with a narrow linewidth of 25 nm.
• the absence of blue-shift in the emission peak with increasing drive currents is in contrast to the commonly observed phenomenon of blue shift in c-plane LEDs working at this wavelength range and similar drive c rent range.
• the linewidth increased almost linearly with the driving current starting from a minimum of 23.5 nm at 20 mA to 27.5 nm at 250 mA. This minimal linewidth broadening with the increase in drive current suggests that the device heating was low in this current regime. The dependence of the output power on the dc drive current was then measured.
• the output power increased sublinearly as the drive current was increased from 10 mA until it saturated at a current level close to 200 mA.
• the saturation of the output power can be attributed to heating effects, thereby causing a reduction in the quantum efficiency.
• the output power at 20 mA forward current was 240 ⁇ W, corresponding to an external quantum efficiency (EQE) of 0.4 %.
• DC power as high as 1.5 mW was measured for a drive current of 200 mA.
• the EQE increased as the drive current was increased, attaining a maximum of 0.42% at 30 mA, and then decreased rapidly as the forward current was increased beyond 30 mA.
• the low EQE for these LEDs can be attributed partially to the poor reflectivity of the (-contact and partially to the "dark" defective window regions of the LEO which do not emit light. It should be noted that the device structure described above constitutes a proof-of- concept, non-optimized device. It is anticipated that significant improvement in EQE can be made by optimization of all aspects of the template/base layers and LED structure.
• nonpolar LED structure includes several key features relevant to the growth and fabrication of a broad range of nonpolar InGaN-based heterostructures and devices. These key features include: 1. Use of a smooth, low-defect-density GaN substrate or template, such as, but not limited to, an HVPE LEO ⁇ -plane or m-plane GaN template. 2. Growth of nonpolar InGaN at a reduced temperature (below ⁇ 900°C) using N 2 carrier gas to enhance In incorporation and decrease In desorption. 3. Growth of the InGaN layers at or near atmospheric pressure (760 Torr) to enhance InGaN film quality and decrease carbon incorporation. 4.
• the preferred embodiment has described a process by which planar, high quality InGaN films and heterostructures may be grown along nonpolar directions.
• the specific example described in the Technical Description section above was for an ⁇ -plane GaN device (i.e. the growth direction was the GaN (1120) direction).
• the base layer for either process could comprise an ⁇ -plane GaN film grown by MBE, MOCVD, or HVPE on an ⁇ -plane SiC substrate.
• Other possible substrate choices include, but are not limited to, ⁇ -plane 6H-SiC, m-plane 6H-SiC, ⁇ -plane 4H-SiC, m-plane 4H-S ⁇ C, other SiC polytypes and orientations that yield nonpolar GaN, ⁇ -plane ZnO, m-plane ZnO, (100) LiAlO 2 , (100) MgAl 2 O 4 , free-standing ⁇ -plane GaN, free-standing AlGaN, free-standing A1N or miscut variants of any of these substrates.
• These substrates do not necessarily require a GaN template layer be grown on them prior to nonpolar InGaN device growth.
• a GaN, A1N, AlGaN, AlInGaN, AlInN, etc., base layer, with or without the incorporation of suitable in situ defect reduction techniques, can be deposited at the beginning of the device growth process.
• the film quality and device performance will be enhanced through the use of a reduced defect-density (i.e., fewer than 1 x 10 9 dislocations/cm 2 and 1 x 10 4 stacking faults/cm "1 ) nitride template/base layer.
• the lateral epitaxial overgrowth process used in this invention achieves defect densities below these levels.
• the preferred embodiment describes an LED structure that contains specifically InGaN and GaN layers.
• the present invention is also compatible with the incorporation of aluminum (Al) in any or all of the layers.
• Al aluminum
• Any or all layers may optionally contain additional dopants, including, but not limited to, Zn, Mg, Fe, Si, O, etc., and still remain within the scope of this invention.
• the capping layer and barrier layers in the device described above are comprised of GaN.
• each of these layers may optionally comprise any nonpolar AlInGaN composition that provides suitable carrier confinement, or in the case of the capping layer, suitable In desorption resistance.
• the thicknesses of the GaN and InGaN layers in the device structure described above may be substantially varied without fundamentally deviating from the preferred embodiment of the invention.
• the layer compositions may be altered to include aluminum and/or boron to alter the electronic band structure. Doping profiles may be altered as well to tailor the electrical and optical properties of the structure. Additional layers may be inserted in the structure or layers may be removed, or the number of quantum wells in the structure may be varied within the scope of this invention.
• the thickness of the ULD GaN capping layer and including an Mg-doped p-type AlGaN electron blocking layer could significantly enhance LED device performance.
• the precise growth conditions described in the Technical Description section above may be expanded as well. Acceptable growth conditions vary from reactor to reactor depending on the geometry of configuration of the reactor. The use of alternative reactor designs is compatible with this invention with the understanding that different temperature, pressure ranges, precursor/reactant selection, V/III ratio, carrier gases, and flow conditions may be used in the practice of this invention.
• the device described herein comprises an LED.
• the present invention is applicable to the general growth of nonpolar InGaN films and structures containing InGaN and should not be considered limited to LED structures.
• Nonpolar strained single quantum well laser diodes could be fabricated using this invention having lower transparent carrier densities than are required for conventional c-plane InGaN-based laser diodes.
• Nonpolar InGaN-based laser diodes fabricated with this invention will also benefit from reduced hole effective masses related to anisotropic strain-induced splitting of the heavy and light hole bands.
• the lower effective hole mass which cannot normally be achieved in c-plane IILnitride devices, will result in reduced threshold current densities for lasing compared to c-plane laser diodes.
• Lower hole effective mass results in higher hole mobility and thus non-polar p-type GaN have better electrical conductivity.
• Electronic devices will also benefit from this invention.
• the advantage of higher mobility in non-polar p-GaN can be employed in the fabrication of bipolar electronic devices like heterostructure bi-polar transistors, etc.
• the higher p-type conductivity in non-polar nitrides also results in lower series resistances in p-n junction diodes and LEDs.
• Nonpolar InGaN channel MODFETs, with reduced radio- frequency (RF) dispersion can now be fabricated that will feature excellent high- frequency performance because of the high saturation electron velocity in InGaN.
• This atmospheric pressure step enhances indium incorporation and decreases carbon contamination in the quantum wells, improving device performance compared to their results.

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• 2005
  • 2005-05-06 EP EP05746303A patent/EP1787330A4/de not_active Withdrawn
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