High sensitivity integrated lateral detection in VCSELs T. Camps, C. Bringer, V. Bardinal, G. Almuneau, C. Amat, E. Daran, J.B. Doucet, P. Dubreuil and C. Fontaine A simple and novel design for power monitoring, integrated in a VCSEL, is presented. A Schottky photodiode, placed close to the VCSEL, enables delivery of a photocurrent of several hundred mA from the lateral emitted light, throughout the whole light-current characteristics. It is shown that the Schottky contact significantly reduces the parasitic current in the cavity. Introduction: Integrated detection in vertical cavity surface emitting lasers (VCSELs) is an important issue to maintain the advantages related to the small dimensions of these laser devices [1]. The solutions appropriate to edge emitter lasers are not relevant for VCSELs, because of their geometry, and especially for arrays. The light power control in VCSELs has therefore been extensively studied, and numerous solutions have been proposed, by an external monitor- ing through a separate discrete photodiode inserted in the packaging [2], or with a photodiode integrated on the wafer surface [3, 4], i.e. in both cases by monitoring the back reflected laser output beam. The integration of a photodetecting section in the vertical structure has also been proposed, but requires dedicated design and additional steps in the fabrication process [5]. We have recently proposed to use an adjacent VCSEL to photodetect the part of guided light emitted in the plane of the cavity [6]. This signal comes mostly from the spontaneous emission escaping laterally from the VCSEL, but nevertheless can be efficiently used to monitor the normal laser output power, since both signals are monotonously correlated. In this Letter, we present a new configuration using a ring-shaped Schottky photodiode (SPD) which enables to maximise the light detection around the VCSEL. Description of device: Fig. 1 represents the microscopic top view of the VCSEL=Schottky detector device and the corresponding AB cross-section. Compared to a standard oxide-confined VCSEL, the mesa etch is stopped at the top – instead of below – of the non- intentionally doped (NID) zone of the cavity. An annular Schottky Ti=Au contact is deposited on this zone. Thereafter, all the results presented in this Letter correspond to a distance (L) of 4 mm, unless mentioned. To isolate the complete device, a second etch is performed outwards of the SPD ring. A B L I D I A DBR DBR N optical waveguide VCSEL L =4 m m detector P I Fig. 1 Top view and AB cross-section of VCSEL=Schottky detector structure The electrical equivalent model of the device is illustrated in Fig. 2. The current applied to the VCSEL (I A ) is divided into two paths, accordingly to the equation: I A ¼ I E þ I P ð1Þ where I E corresponds to the effective current passing through the PIN VCSEL diode, and I P to the parasitic current due to the conduction in the NID layer channelling towards the SPD electrode. Measurements demonstrate that the conduction in this layer is purely resistive and can thus be represented by a resistance in our model (R P ). In the same way, the detected current through the SPD (I D ) can be defined by: I D ¼ I PH þ I P ð2Þ where I PH corresponds to the photocurrent related to the lateral detection of the in-plane spontaneous emission. Moreover, the Ti=Au contact on the Al 0.3 Ga 0.7 As cavity layer (NID) forms a Schottky barrier, adding a reverse voltage (V S ). To prevent any change of the VCSEL characteristics (i.e. I E ’ I A ) and to detect more efficiently the lateral emission (i.e. I D ’ I PH ), the parasitic current I P has to be minimised; it can be expressed as the following: I P ¼ V E  V S R P ð3Þ In reverse conduction, the Schottky contact induces a potential (V S ) very close to the voltage across the VCSEL PIN diode (V E ), which reduces the voltage drop across R P and consequently limits the parasitic current I p . Increasing the distance L reduces this parasitic current, but also involves a significant reduction of the detectable light intensity. Measurements: Fig. 3 represents the evolutions of the applied voltage (V A ), the detected current I D and the emitted power P L against applied current I A . R P I P V S I D A I PH lateral detector VCSEL L R top I E V E I A V A Fig. 2 Equivalent electrical model of device 0 0.5 1.0 1.5 I D , mA I A , mA 0 10 20 30 40 50 L, mW 0 0 2 1 4 2 6 3 V A ,V Fig. 3 L(I) and V(I) characteristics of VCSEL and I D (I) characteristic of SPD 0 0.5 1.0 1.5 current, mA V A ,V 1.5 3.0 2.5 2.0 560 A m 1.39 V 1.74 V 2.69 V I PH I D I A L L, mW 10 -6 10 -3 10 -2 10 -1 10 -5 10 0 10 -4 10 1 Fig. 4 Photocurrent evolution illustrating different regimes in VCSEL operation The evolutions of the V A (I A ) and P L (I A ) characteristics are standard, but the detected current I D (I A ) curve exhibits a peculiar shape. The signal detected by the SPD can be attributed to spontaneous emission, since this latter is distinguishable by a wide electroluminescence spectrum [6, 7]. When the gain clamping is expected at the laser threshold, a slope break on the I D curve at low injection is observed [8]. Further, when thermal rollover appears at high injection, the curve is marked by a second slope break. Moreover, the increasing monotonous variation of I D between the threshold and the extinction makes this lateral detection suitable for integrated VCSEL power monitoring. For the low values of I A , the voltage drop in the resistance R top (top Bragg mirror) is reduced, consequently the effective voltage V E supported ELECTRONICS LETTERS 3rd February 2005 Vol. 41 No. 3