Abstract:
An apparatus for evaluating an associated semiconductor sample having two electrically distinct regions with a junction region disposed therebetween includes a laser for injecting carriers into a sample region, an electrical bias for impressing electrical fields on the sample, and a detector for detecting luminescence. A second laser is provided for injecting carriers into a second sample region opposite the first region. A method includes the steps of: optically generating carriers in a region, generating a drift field in the region that effectuates carrier drift toward the junction, and measuring the optical radiation generated by carrier recombination in the junction region. Preferably, the method also includes optically generating carriers in a second region and generating a drift field in the second region that effectuates carrier drift toward the junction. Typically, the two drift fields are generated together by applying voltage between the two regions.

Description:
BACKGROUND OF INVENTION 
     The invention relates to semiconductor materials and devices characterization or evaluation, and more particularly to the electrical and optical characterization of light emitting diode (LED) device structures. The invention will be described with particular reference to the characterization of quantum well-based LED structures. However, the invention is not so limited, but will also find application in optical and optoelectronic evaluation of p/n junctions, semiconductor laser structures, and the like. 
     The prior art discloses semiconductor characterization using a very broad range of experimental techniques. Semiconductor materials and devices are commonly characterized or evaluated using x-ray diffractometry, photoluminescence, cathodoluminescence, and electroluminescence, among many other techniques. In the case of optoelectronic devices which convert electrical energy to optical energy and/or vise versa, methods which excite luminescence in the material are particularly useful. In photoluminescence, excess carriers (excess electron-hole pairs) are photoexcited by exposure to a sufficiently intense light source, and the luminescence emitted as these photoexcited carriers recombine is measured. The luminescence can be measured spectroscopically and/or as a function of time after the light source is turned off. Cathodoluminescence is similar to photoluminescence except that the excess carriers are generated by exposure to an electron beam rather than by exposure to light. 
     For evaluating a light emitting diode (LED) device structure, the electroluminescence behavior is of greatest interest, as the finished LED device functions through electroluminescence. Electroluminescence is similar to photoluminescence and cathodoluminescence, except that in electroluminescence the excess carriers are electrically injected. In the case of an LED, the electrical injection of carriers into the optically active p/n junction region is achieved by forward biasing the p/n junction. However, electroluminescence is not equivalent to photoluminescence, because the electroluminescence behavior of a sample is determined by a number of factors, such as the optical properties of the optically active layers, the electrical transport properties (e.g., conductivity) of the p-type and n-type regions, and the properties of the electrical contacts through which the electrical biasing is applied. Some of these factors, particularly those relating to transport, can produce different effects on the electroluminescence versus the photoluminescence. It is to be appreciated that in photoluminescence, both the excess conduction electrons and the excess holes are typically injected into the same side of the junction, whereas in electroluminescence the injection of electrons and holes are on opposite sides of the junction. 
     An important class of LED&#39;s are epitaxially grown double heterostructure-based LED&#39;s (DH-LED&#39;s). In these devices, the simple doping junction of the standard p/n diode LED is replaced by an active region containing luminescent material, and with an energy gap less than that of the surrounding p and n type materials. The active region is preferably sandwiched between the p-type and n-type regions of the DH-LED. Light emission in a DH-LED is through the radiative recombination of electrically injected excess carriers inside the active layer. The active layer of a DH-LED defines a potential well. If the dimension of the active layer is less than about 10 nm, then the double heterostructure is called a quantum well. Multiple quantum wells can exist in the active layer of a heterostructure LED. 
     The active region of a DH-LED serves, in addition to physically hosting the luminescent material, as a carrier confinement region that confines carriers inside the active layer or quantum wells. If an electron-hole pair exists inside a potential well, the likelihood of recombination increases as the width of the well decreases. This is simply because the electron is physically closer to the hole in a narrow potential well than in a wider potential well. 
     The electroluminescence of DH-LED&#39;s and quantum well-based LED&#39;s is further complicated by the additional structural complexity. The electroluminescence can be affected by factors such as the effectiveness of the carrier confinement, interfacial defects, impurities at the quantum well boundaries or inside the quantum wells, the relative confinement of conduction electrons versus holes (typically determined by the conduction band and valence band offsets at the interface between the quantum well and the barrier material), crystalline quality of the quantum wells, atomic interdiffusion at the quantum well interfaces, and the like. It will again be appreciated that these effects can be different for electroluminescence versus photoluminescence. 
     Commercial LED wafers are typically tested at the wafer level using photoluminescence. However, it is generally known to the art that high photoluminescence efficiency is a necessary but not a sufficient test of an LED wafer. A wafer that exhibits poor active layer photoluminescence properties will usually also exhibit poor electroluminescence behavior, translating into poor LED&#39;s fabricated therefrom. However, a wafer with high photoluminescence efficiency may or may not produce high electroluminescence efficiency and hence good LED&#39;s, because of differences between the electroluminescence and photoluminescence processes as discussed above. Thus, there remains an unfulfilled need for improved screening of LED wafers at the wafer level. 
     The prior art also does not teach effective means for separating out the various components of the electroluminescence signal. Poor electroluminescence or LED behavior can result from failure at any layer of the LED structure, or from problems introduced during LED fabrication. The prior art teaches generating a matrix of varying sample growth conditions and fabrication steps and analyzing the matrix, e.g. by fabricating LED&#39;s therefrom, in the hope of correlating the matrix parameters with changes in the LED behavior or the electroluminescence. This approach has several disadvantages. First, it is expensive in terms of personnel time, equipment load, and source materials. Second, it is highly subjective. Misleading results can easily be obtained if elements of the sample matrix include unknown variations, e.g. differences in doping level between samples for a layer which has the same nominal doping level for all the samples of the matrix. Even if an unintended matrix variation is recognized, e.g. through doping concentration measurements, it still can be difficult or even impossible to correct the data therefor. 
     In view of these disadvantages, it would be useful to have an improved characterization method that preferably is performed at the wafer level and more closely resembles the physical mechanisms of electroluminescence and LED operation, and that has the ability to independently evaluate for a single sample the relative contributions or effects on the electroluminescence characteristics of the various sample regions such as the active region including the quantum well or wells, the p-type material region, the n-type material region, and the electrical contacts. 
     The present invention contemplates such an improved characterization or evaluation method and apparatus. 
     SUMMARY OF INVENTION 
     In accordance with one aspect of the present invention, an apparatus for evaluating an associated semiconductor sample is disclosed. The associated sample has a first electrically distinct region and a second electrically distinct region, and further has a junction region disposed therebetween. The evaluation apparatus includes a stage for mounting the semiconductor sample. A first laser has a wavelength tuned to photogenerate carriers in the first electrically distinct region. An electrical biasing means is provided for impressing an electrical field whereby at least some photoexcited carriers are influenced to drift toward the junction region. The photoexcited carriers are holes from the p-side and electrons from the n-side. In this manner, instead of injecting electron-hole pairs from one side through thermal diffusion, electrons and holes are injected from different sides as they would be in an actual LED. An optical detector is provided, whereby luminescence generated by recombination of the photoexcited carriers in the junction region is detected. 
     Preferably, the apparatus includes a translation means for relatively translating the laser and the sample whereby the laser beam is scanned across the sample. A second laser is preferably disposed on the opposite side of the sample with respect to the first laser. The second laser has a wavelength tuned to photogenerate carriers into the second electrically distinct region. Preferably, the first laser has a wavelength tuned to a first energy approximately corresponding to the energy band gap of a material comprising the first electrically distinct region, while the second laser has a wavelength tuned to a second energy approximately corresponding to the energy band gap of a material comprising the second electrically distinct region. Optionally, the two wavelengths can be the same, i.e. the same laser beam is split to serve as both the first laser and the second laser. 
     In one application, the associated sample has at least one potential well in the junction region. The optical detector preferably has a detection wavelength range which essentially includes the active layer luminescence. In a more specific application, the first region of the associated sample includes n-type gallium nitride, the second region of the associated sample includes p-type gallium nitride, and the active layer of the associated sample includes an alloy of indium gallium nitride. In this case, the first laser and the second laser preferably have wavelengths less than 365 nm to provide adequate absorption by the semiconductor. Preferably, at least one of the group including the first laser and the second laser is a tunable wavelength laser. 
     In accordance with another aspect of the present invention, a method for characterizing an associated semiconductor sample is disclosed. The associated sample has a first electrically distinct region and a second electrically distinct region, and further has a junction region disposed therebetween. The characterization method includes the steps of optically generating carriers in the first electrically distinct region, generating an externally applied drift field in the first region that effectuates a drifting of the optically generated carriers in the first electrically distinct region toward the junction region, and measuring the optical radiation generated by radiative recombination of the optically generated carriers in the junction region. 
     Preferably, the characterization method also includes optically generating carriers in the second electrically distinct region, and generating an externally applied drift field in the second region that effectuates a drifting of the optically generated carriers in the second electrically distinct region toward the junction region. Typically, the step of generating an externally applied drift field in the first region and the step of generating an externally applied drift field in the second region are performed together by applying a voltage between an electric contact that electrically contacts the first electrically distinct region and an electric contact that electrically contacts the second electrically distinct region. 
     In the step of generating an externally applied drift field in the first region, an electric drift field described by a field vector E is generated. Preferably, in the step of optically generating carriers in the first electrically distinct region, the optically generated carriers are substantially generated within a distance d=μτ|E| of the junction region, where μ is the drift mobility of the optically generated carriers in the first material, and τ is the lifetime of the optically generated carriers in the first material. Under these conditions, the fraction of the optically generated carriers which enter the junction region is approximately 1/e. 
     The method preferably further includes estimating quantitatively the volume recombination rate in the junction region based on the step of measuring the optical radiation generated by radiative recombination of the optically generated carriers in the junction region; estimating quantitatively the volume density of optically generated carriers in the first electrically distinct region; and estimating quantitatively the electroluminescence efficiency based upon the volume recombination rate and the volume density of optically generated carriers. 
     In the above method, the magnitude of the drift field produced in the step of generating an externally applied drift field in the first region is preferably sufficiently low such that the number of carriers electrically generated is negligible compared to the optically generated carriers. 
     In accordance with yet another aspect of the present invention, A method for characterizing a light emitting diode (LED) structure sample is disclosed. The sample has an n-type region and a p-type region with a junction region disposed therebetween. Carriers are optically generated in the n-type region by light impingement thereon. Carriers are optically generated in the p-type region by light impingement thereon. The optical radiation generated by radiative recombination of the optically generated carriers in the junction region is measured. 
     Preferably, the method further includes electrically biasing the junction and to effectuate a drifting of the optically generated carriers toward the junction region. 
     Preferably, the method further includes optically chopping the impinging light with an optical chopper, detecting the optical radiation with an optical detector, and measuring the optical detector signal at the optical chopping frequency using a lock-in amplifier that is in operative communication with the optical chopper and the optical detector. 
     Preferably, the method further includes repeating the generating, biasing, and measuring steps at a plurality of wavelengths of the at least one optical source, and estimating transport properties of the at least one region therefrom. 
     Preferably, the method further includes repeating the generating, biasing, and measuring steps at a plurality of intensities of the at least one optical source, and estimating the effects of high injection levels from the measuring. 
     One advantage of the present invention is that it permits separately probing the effects of transport in the p-type and n-type regions, artifacts due to the electrical contacts, and properties intrinsic to the active region. 
     Another advantage of the present invention is that it permits spatial profiling of the LED heterostructure across the wafer. 
     Another advantage of the present invention is that it permits depth-dependent profiling into both the n-side and the p-side of the LED structure. 
     Yet another advantage of the present invention is that it facilitates photoexcited electroluminescence whereby simultaneous excitation from the front and the rear of the wafer is performed. 
     Still yet another advantage of the present invention is that it provides a wafer level characterization method that is closer to the physical behavior of an operating LED versus prior art wafer level characterization methods. 
     Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. 
     FIG. 1 is a drawing of an experimental apparatus according to one embodiment of the invention. 
     FIG. 2 is a drawing of the physical processes which occur inside the sample during a method carried out in accordance with the apparatus drawn in FIG.  1 . 
     FIG. 3 is a drawing of an experimental apparatus according to another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 1, an experimental apparatus  10  is described in accordance with one embodiment of the invention. The experimental apparatus  10  operates upon an associated LED sample  12 . The associated sample  12  includes a substrate  14  and typically a plurality of semiconductor layers  16 ,  18 ,  20  usually grown epitaxially thereon. In FIG. 1, an exemplary gallium nitride (GaN) based LED sample  12  is shown, which includes a sapphire substrate  14 , a p-type GaN region  16  and an n-type GaN region  18 . Sandwiched between the p-type GaN region  16  and the n-type GaN region  18  are a plurality of In x Ga 1−x  N quantum wells  20  which comprise the active region of the LED device. “Quantum wells” are InGaN layers typically around 10 nm thick or less. Although four quantum wells  20  are drawn, it is to be appreciated that GaN QW-LED&#39;s with as few as one InGaN layer are feasible, and the layer need not be a quantum well. The In x Ga 1−x N layers are alloys of InN and GaN, and the mole fraction of InN in the quantum well or wells, denoted by x in the formula In x Ga 1−x N, strongly affects the LED emission peak wavelength, through the band gap of the In x Ga 1−x N material, and through an incompletely understood mechanism involving physical segregation of InN from GaN in the alloy. Additional layers, such as nucleation buffer layers (not shown) are also optionally incorporated to facilitate the crystal growth or for other reasons related to the design and performance of the LED. Of course, the invention is not limited in application to the exemplary GaN-based LED structure drawn in FIG. 1, but will also find application in the characterization of a wide range of LED structures comprising various materials and various layer combinations, as well as in the characterization of other optoelectronic materials and devices such as semiconductor laser device structures. 
     With continuing reference to FIG. 1, a p-type electrical contact  22  is formed which contacts the p-type region  16 . A series of vias  24  are etched through the p-type region  16  and the quantum wells  20  thereby exposing portions of the n-type region  18 , and n-type electrical contacts  26  are formed therein. It will be appreciated that the p-type electrical contact  22  preferably includes a region which is essentially transparent with respect to the photoexcitation light source which will be described next. In the illustrated embodiment of FIG. 1 this transparent area includes the entire contact  22  area. In a preferred embodiment for a group III-nitride LED, a nickel oxide/gold or cobalt oxide/gold contact is used for the p-type contact  22 . This contact is preferably formed by depositing 5-10 nm of nickel followed by 5-10 nm of gold and annealing the contact at 450-600° C. for 5 minutes in air. The thickness of the gold is sufficiently thin in the region of photoexcitation light impingement to ensure good transparency. Of course, the described contact is exemplary only, and other transparent contacts may be used instead. The vias  24  and the contacts  22 ,  26  are preferably formed using standard semiconductor processing techniques well known to those skilled in the art. 
     It will be appreciated that the fabrication steps just described can be performed at the wafer level in an essentially non-destructive manner. Depending upon the conductivity of the n-type layer or of the underlying substrate, the contact vias are optionally restricted to peripheral areas of the wafer so that the central wafer areas remain undisturbed and available for subsequent commercial LED device fabrication. In one embodiment, the Ni/Au oxidized contact is the first step of the LED fabrication process. In another embodiment, the Ni/Au oxidized contact is selectively removable using standard etching methods that are well known to those of ordinary skill in the art. It will also be appreciated that for conductive substrates such as silicon carbide or gallium nitride, the contact vias  24  are optionally replaced by direct electrical contact to the conductive substrate, further facilitating wafer-level testing. 
     With continuing reference to FIG. 1, the experimental apparatus  10  includes a first light source  30  which applies a first photoexcitation light  32  to the p-type region  16  through the transparent region of the p-type electrical contact  22 . In a preferred embodiment, the first light source  30  is a laser with a well defined lasing wavelength, although lamp systems with appropriate conditioning optics such as wavelength-selective filters can be substituted therefor. Preferably, a second light source  34  applies a second photoexcitation light  36  in the n-type region  18 . It will again be appreciated that the second photoexcitation light  36  should pass through the substrate  14  without excessive optical attenuation. In the exemplary case of a sapphire substrate, this transparency condition is met for a preferred wavelength range around approximately 365 nm or shorter (sapphire is transparent down to 200 nm) which is appropriate for photoexcitation of the GaN regions  16 ,  18 , as well as for the wavelength range around approximately 365 nm or longer which is appropriate for photoexcitation of typical In x Ga 1−x N quantum wells. For partially opaque substrates, the substrate is optionally thinned in the region of interest to obtain sufficient transparency. 
     With continuing reference to FIG. 1, the experimental apparatus  10  also includes a biasing means  40  for applying a variable electrical bias to the sample  12 . In the apparatus drawn in FIG. 1, the biasing means is a d.c. voltage source  40 , such as a battery with a variable resistor, a commercial d.c. power supply, a custom-built power supply, or the like. The biasing means  40  is preferably connected to the p-type contact  22  by wiring  42 , and to the n-type contacts by wiring  44 . In one embodiment of the invention, the DC bias is applied just below threshold, so that the only spatial region of significant current flow through the active layer is near the photoexcited volume. It will be appreciated that the biasing means can take various other forms. 
     The experimental apparatus  10  also includes an optical detector  46  which detects luminescence generated by the associated sample  12  under the influence of the experimental apparatus  10 . The detector  46  can be a photomultiplier tube, a photodiode, a diode array, or the like, and also preferably includes a light-collecting lens  48 , optical fiber coupling (not shown), appropriate drive electronics (not shown), and a dispersive component such as a monochromator, spectrograph, or the like. 
     With continuing reference to FIG. 1, and with further reference to FIG. 2, an exemplary method implemented by the experimental apparatus  10  is described. The first photoexcitation light  32  impinges on the p-type region  16 . For a properly selected wavelength, the photoexcitation light  32  is absorbed primarily in the p-type region  16 . The photon absorption process photogenerates electron-hole pairs. The wavelength is selected to position the photoexcited electron-hole pair distribution essentially within a certain depth range, which is determined by the absorption spectrum of the material or materials comprising the p-type region  16 . In the p-type material  16 , photoexcited majority carrier holes are represented in FIG. 2 by an exemplary hole  50 . Similarly, the second photoexcitation light  36  having a wavelength selected to be absorbed in the n-type region  18  will generate electron-hole pairs therein, and the photoexcited electrons are represented in FIG. 2 by an exemplary electron  52 . It will be appreciated that this excess carrier injection arrangement differs fundamentally from that of conventional photoluminescence, because in conventional photoluminescence one or the other of the light sources is absent, and so transport in one of the cladding layers is not investigated. The optical injection arrangement shown in FIG. 2 more closely resembles the actual operation of the final LED device. 
     The biasing means  40  generates a voltage  54  across the sample as shown in FIG.  2 . The voltage  54  generates electric fields E p , E n  in the bulk p-type  16  and n-type  18  regions, respectively. In FIG. 2, the LED  12  is placed in forward bias corresponding to the typical biasing polarity of an operational LED. However, methods employing the apparatus  10  in which a reverse bias is applied are also contemplated. It will be appreciated that the polarity of the forward bias voltage  54  and of the corresponding impressed electric fields E p , E n  are such that both the photoinjected holes  50  and the photoinjected electrons  52  are influenced to move toward the junction region  60 . Within the junction region, an active layer or one or more quantum wells  20  exist. In FIG. 2, a single quantum well  20  is shown for simplicity. The electron and the hole drift under the influence of the applied voltage  54  along paths  64  and  66  for the hole  50  and the electron  52 , respectively. The holes  50  and the electron  52  preferably recombine inside the quantum well and emit a photon represented by a ray  70  which contributes to the sample luminescence collected by the collecting lens  48  and measured by the detector  46 . 
     For a sufficiently high applied voltage the electrically injected carriers in an LED will be more numerous than the photoexcited carriers. Here, however, lower voltages  54  are typically applied, so that the applied voltage  54  is merely influencing the photoexcited carriers to move toward the junction. Lower applied voltages typically reduce non-ohmic contact behavior so that the sample response is more characteristic of the semiconductor layers rather than the contacts. The local threshold voltage in an LED wafer is depressed by the presence of photoexcited carriers, particularly in the p-type material. In one embodiment, the forward bias  54  is set just below threshold, so that the luminescence will be generated predominantly in the photoexcited volume. 
     In one exemplary embodiment in which the associated LED structure is a simple p/n homojunction without quantum wells (sample not shown), an appropriate electric field magnitude can be estimated based on the drift velocity v=μ|E| where μ is the drift mobility of the optically generated carriers in the material and |E| is the magnitude of the electric field impressed by the voltage  54 , e.g. the field E p  or the field E n . The average distance a carrier moves before recombining is d=vτ=τμ|E| where τ is the carrier lifetime in the material. Based upon such transport estimates, the fraction of photoinjected carriers expected to reach the junction region is estimated. Furthermore, the volume density of photoinjected carriers is obtained from the light intensity and the absorption characteristics of the material. The volume (or areal) recombination rate in the junction region is estimated from the luminescence intensity measured by the detector  46  along with geometrical factors. The electroluminescence efficiency is obtained from the volume (or areal) recombination rate and the volume density of optically generated carriers, taking into account the fraction of carriers which reach the junction region. 
     The above-described homojunction LED calculations are exemplary only. Particularly in more complex heterojunction-based LED&#39;s such as the exemplary GaN LED  12  of FIG. 1, the effectiveness of the bias voltage  54  in driving carriers into the active region (e.g., the p-type region  16  or the n-type region  18 ) will depend upon many factors, such as contact resistance, doping levels, potential barriers in the active region, and et cetera. In FIG. 2, exemplary potential barriers E b  can impede injection of carriers into the quantum well  20 . Even in the case of a homojunction LED complexities can arise due to impurities, non-uniform dopant distributions, and the like. Because of the complexity of a typical LED samples, quantitative calculations are often impractical in practice. However, by varying selected operational parameters of the apparatus  10  while holding other operational parameters constant, as described next, the individual electroluminescence contributions of the various structural regions of the sample can be separately and independently examined. 
     With continuing reference to FIGS. 1 and 2, a typical evaluation or characterization of an associated sample such as the exemplary GaN LED  12  using the exemplary method and apparatus of FIGS. 1 and 2 is described. For some measurements, the first and second light sources  30 ,  34  of the apparatus  10  are preferably wavelength-tunable sources. By obtaining data for several wavelengths of the first light source  30  while holding the other operational parameters of the apparatus  10  constant (e.g., constant electrical bias  54 , constant intensity and wavelength for the second light source  34 ), the carrier injection from the p-type region  16  is investigated. Varying the wavelength varies the depth of the photoinjected carriers, so that the electroluminescence variation with the wavelength of the first light source  30  correlates with the carrier transport properties of the p-type region  16 . Such measurements also can provide information about potential barriers E b  which may be impeding carrier injection from the p-type region  16  into the junction region  60 . In an analogous manner, varying the second light source  34  while holding the other operational parameters constant probes carrier injection from the n-type region  18 . 
     In another characterization aspect, by increasing the intensity of light sources  30 ,  34  together or independently, the effects of high injection levels into one or both regions  16 ,  18  is probed essentially independently from contact behavior artifacts which are usually produced at high current levels. In yet another variation, keeping the light sources constant while varying the applied voltage  54  probes factors whose effect on the electroluminescence correlate with high bias voltage  54 . For example, the effects of high contact resistances at the contacts  22 ,  26  can be investigated in this manner. 
     For sufficiently long wavelengths, the absorption typically takes place predominantly inside the quantum wells. Considering the exemplary GaN QW-LED  12 , the p-type and n-type regions  16 ,  18  are comprised of GaN while the quantum wells are comprised of In x Ga 1−x N material which has a lower-band gap. Under selected long wavelength conditions, absorption occurs primarily in the In x Ga 1−x N and so transport through the p-type and n-type regions  16 ,  18  is essentially irrelevant. The luminescence properties of the active region as a function of applied electrical bias can thus be evaluated directly and independently from the cladding regions  16 ,  18 . 
     In the apparatus  10 , the associated sample  12  is preferably mounted on a translation stage (not shown) whereby the sample  12  is moved laterally with respect to the light sources  30 ,  34 . In this way, lateral inhomogeneities can be probed. Lateral translation is often useful for detecting variations in quantum well thickness across the wafer, for example. Material damage near an alloyed contact can also be evaluated by scanning the injection region toward the contact. 
     In the apparatus embodiment of FIG. 7, two independent light sources  30 ,  34  are used to inject carriers into the p-type region  16  and the n-type region  78 , respectively. This approach enables the use of different wavelengths for exciting the two sides of the junction. The two excitation beams optionally have different coordinates in the horizontal plane so as to probe lateral diffusion and drift effects; also the two excitation beams are optionally pulsed, with pulse width short compared to carrier lifetime so as to probe temporal drift, diffusion, and recombination effects; furthermore the pulses need not be coincident, so as to independently probe temporal drift, diffusion, and recombination effects in the n and p type material. However, in many cases the two sides are comprised of the same material, e.g. GaN. For such samples, separate wavelength adjustment capability for each of the two sides of the junction may not be particularly advantageous. 
     With reference to FIG. 3, a second apparatus embodiment  100  of the invention is described. The embodiment is shown acting on the same exemplary GaN-based sample  12  as is shown in FIG.  7 . The biasing arrangement, comprising variable biasing means  140  and wiring  742 ,  144  is preferably essentially unchanged from the embodiment of FIG.  1 . However, the two light sources  30 ,  34  of the embodiment of FIG. 1 are replaced in the embodiment of FIG. 3 by a single light source  130  with a beam splitter  132 , several mirrors  134 , and two variable intensity attenuators  136  which can take the form of filter wheels, removably insertable neutral density filters, shutters, or the like. The single light source  130  is preferably an adjustable wavelength light source. 
     The light detection is preferably by an optical detector  146  and a light collecting lens  148 , both of which are similar to the corresponding components of the first apparatus of FIG.  1 . However, because a single light source is used, the apparatus of FIG. 3 optionally includes a lock-in detection sub-system including an optical chopper  150  and a lock-in amplifier  152  in operative communication with the optical chopper  150  and the optical detector  146 . As is known to those skilled in the art, use of lock-in detection greatly increases the signal-to-noise ratio of the detected luminescence signal. Of course, other signal detection sub-systems can be substituted therefor. 
     The methods described with respect to the apparatus of FIG. 1 are also generally compatible with the apparatus of FIG.  3 . However, with the apparatus of FIG. 3 the wavelength of light impinging on the p-type region  16  and the n-type region  18  cannot be independently varied. The relative light intensities are, however, independently variable through the two variable attenuators  136 , and so the magnitude of the carrier injection into the two regions  16 ,  18  can be independently controlled. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.